Prep the steak: With paper towels, pat the steak dry. Place in a large ziptop bag or large bowl, with the salt, pepper, balsamic vinegar, and oil. Seal the bag (if using), removing as much air as possible. Turn to coat the steak. Set aside at room temperature for 30 minutes, or refrigerate overnight.: The room fills with a dryer warmth as you pat the steak dry with paper towels, which helps the surface brown more effectively when exposed to heat. When you combine the steak with the measured kosher salt , ground black pepper , 2 tablespoons of balsamic vinegar , and 1 tablespoon of extra-virgin olive oil in a large ziptop bag or bowl, the meat picks up a glossy sheen and a faint tangy scent. Gently pressing out excess air in the bag helps the marinade cling to the steak , but because this is a quick marinade, the idea is to lightly coat rather than fully saturate. Letting the meat sit at room temperature for 30 minutes relaxes muscle fibers so the steak cooks evenly, and refrigerating overnight deepens flavor if you plan ahead. Common mistake to avoid, don't over marinate acidic mixtures for too long, as they can break down the meat and create a mushy texture.
Meanwhile, cut the onion into 1/2-inch thick rings. Lightly brush each side with olive oil. Cut the cherry tomatoes in half.: As you slice the red onion into 1/2-inch thick rings, the kitchen gets a sharp, sweet aroma; brushing both sides with extra virgin olive oil prevents sticking and helps the natural sugars caramelize under heat. Halving the cherry tomatoes releases their juicy perfume and exposes their seeds, which will burst with flavor on the plate. This prep is about controlling moisture and creating surfaces that will char or soften evenly. A typical misstep is slicing uneven rings, which creates inconsistent grilling and burnt edges on thinner pieces.
Make the dressing: In a medium bowl or large measuring cup, whisk together the balsamic, oil, Dijon, honey, salt, and pepper.: Whisking together 1/4 cup balsamic vinegar , 3 tablespoons extra virgin olive oil , 1 teaspoon Dijon mustard , 1 teaspoon honey , 3/4 teaspoon kosher salt , and 1/8 teaspoon ground black pepper yields a glossy vinaigrette that smells bright and faintly sweet. The mustard helps emulsify the mixture so the oil does not separate, creating a smooth coating for the greens. Taste and adjust, keeping in mind the vinaigrette should be lively but not overpower the steak or blue cheese . Avoid adding too much salt at once, since the cheese and steak will contribute additional seasoning when tossed.
Cook the steak and onion: Heat the grill over medium-high heat (425 to 450 degrees F) or heat the oven’s broiler. If using the broiler, line a rimmed baking pan with foil. Remove the steak from the marinade, allowing as much excess marinade as possible to drip away. Grill or broil the onion and steak, until the steak is medium-rare and the onion is charred on both sides—3 to 4 minutes per side depending on its thickness, until the steak reaches 130 degrees F on an instant read thermometer. (If you like your steak more well done, cook it 1 to 2 minutes more, but note that it will be tough).: Preheating to medium high, around 425 to 450 degrees F, ensures a hot cooking surface that sears meat and chars onion rings quickly. If using the oven broiler, line a rimmed baking pan with foil to catch drips and make cleanup simple. A properly heated grill will give a satisfying sizzle when the steak hits the grates, creating a crust that locks in juices. The wrong move here is rushing to cook on a cool surface, which leads to gray, steamed meat rather than a flavorful sear.
Place the steak on a cutting board and let rest at least 5 minutes.: As the steak and onion meet intense heat, you should hear a steady sizzle and see the edges begin to caramelize; charred marks on the onion add smoky sweetness while the steak develops a richly browned crust. Grill or broil each side 3 to 4 minutes depending on thickness, aiming for an internal temperature of 130 degrees F for medium rare. Resting after cooking is critical to let juices redistribute; skipping this will cause the steak to lose more juices when sliced. If you prefer more done meat, cook an extra 1 to 2 minutes per side, but note that overcooking tougher cuts can make them less tender.
In a large bowl, place the mixed greens, arugula, halved tomatoes, half of the cheese, and the grilled onions (separate the rings as you add them).: After removing the steak from heat, place it on a cutting board and let it rest for at least 5 minutes, during which aromas settle and surface juices retract slightly. Resting produces a juicier texture when you slice, and the meat will feel firmer to the touch as it carries residual heat. Avoid the urge to slice immediately, as cutting too soon lets flavorful juices run out and leaves you with drier slices on the plate.
Add enough dressing over the top to moisten, then toss to coat. Divide the salad between individual plates, or place in the center of a large serving plate.: In a large bowl combine 3 cups mixed greens , 3 cups arugula , the halved cherry tomatoes , half of the crumbled blue cheese , and the grilled onions separated into rings. Tossing these ingredients together releases the tomatoes' juices onto the leaves, creating a light dressing base even before vinaigrette is added. The visual contrast here is important, aim for an even distribution so each plate gets a mix of textures. A common error is overdressing at this stage, which wilts the greens prematurely.
VERY thinly slice the steak across the grain (about 1/4-inch wide). Serve on top of or alongside the salad. Sprinkle on the remaining blue cheese. Enjoy!: Drizzle enough of the vinaigrette over the salad to moisten the leaves, then toss to coat evenly, making sure the dressing clings to leaves and tomatoes without pooling. The emulsion should lightly coat the salad, offering glossy sheen and balanced acidity. When tossing, lift from the bottom to avoid crushing the components, and stop when the colors look vibrant and slightly glossy. Too much dressing will drown the delicate textures and mask the steak's flavor.
Slice the steak thinly: Using a sharp knife, very thinly slice the steak across the grain into roughly 1/4-inch wide ribbons, which keeps each bite tender and easy to chew. The cross grain cut shortens muscle fibers and makes the meat feel more tender on the palate. Arrange slices on top of or alongside the salad so warm meat briefly warms the leaves without making them soggy. A frequent mistake is cutting with the grain, which makes pieces chewier and tougher.
Finish and serve: Scatter the remaining crumbled blue cheese over the plated salad and add additional cracked ground black pepper or kosher salt if desired. The combination of warm steak , charred onion , and cool greens creates a pleasing temperature contrast that reads as both hearty and refreshing. Serve immediately to preserve texture, and expect compliments. Do not let the salad sit too long after dressing, or the greens will begin to lose their crispness.