Add Spice Mix seasonings to a small bag or sealable container; whisk and set aside.: You will notice aromatic, earthy notes as the powdered spices come together which helps ensure even distribution later, and this step prevents clumping when the spice mix hits wet ingredients. One common mistake is skimping on whisking which leaves pockets of strong flavor; thoroughly combine until the color is uniform and the aroma is cohesive.
Add all Marinade Ingredients (except steak) to a large freezer bag and whisk to combine. Add 3 tablespoons Spice Mix to Marinade and whisk to combine. Add steak, squeeze out excess air, seal and turn to evenly coat. Marinate in the refrigerator 4-12 hours (the longer the better).: The marinade will smell tangy and sweet with savory depth, and fully coating the flank steak lets the acids and aromatics penetrate. A typical pitfall is not removing air from the bag which limits contact between meat and marinade, so press out as much air as possible and flip the bag a few times during marinating to redistribute the liquid.
While steak is marinating, prepare pico de gallo and other toppings aside from guacamole and refrigerate.: Freshly chopped Roma tomato and crisp red onion will smell bright and vegetal, and chilling the toppings lets flavors meld. Avoid making guacamole too early or it will brown; keep the avocado components separate until closer to service to preserve color and texture.
When ready to grill, mix remaining Spice Mix with 2 tablespoons olive oil to create a wet Spice Rub.: The oil blooms the spices, releasing fragrant oils and creating a paste that adheres to the meat for a better sear. A mistake here is using too little oil which prevents an even rub; aim for a cohesive paste that spreads smoothly over the steak.
Discard marinade and pat steaks dry with paper towel. Rub steak evenly all over with Spice Rub and let rest at room temperature for 30 minutes. I use this time to make guacamole (recipe to follow).: Patting dry reduces steaming and allows the surface to caramelize, and resting brings the steak closer to even cooking temperature. Do not skip the drying step or the meat will not form a proper crust; you want a dry surface to achieve those desirable charred notes.
Grease and preheat outdoor grill or indoor grill pan to high heat. Add steak and turn down to medium-high. Grill steak for 5-6 minutes per side, turning once, for medium (recommended 145 degrees F). For medium-well, continue cooking steak over indirect flame for an additional 5-10 minutes per side, depending on thickness. *If you like your steaks rare then use HIGH heat so you can get the charred outside in less time.: As the steak hits the hot grates you will hear an immediate sizzle and smell a toasty, caramel aroma which signals the Maillard reaction. Using direct high heat first gives a crisp exterior, then moving to medium high ensures an even cook. Overcrowding the grill is a common error that drops temperature and prevents searing, so work in batches if needed.
Remove steak and let rest 10 minutes before chopping into small pieces.: Resting allows juices to redistribute, resulting in moist, tender pieces rather than a dry, leaking steak. You will notice the meat relax and become easier to slice, a clear sign you are ready. Cutting too soon is a frequent mistake that causes the juices to run out; patience here pays off in texture.
Warm tortillas in the microwave, in a skillet or char on the grill or open gas stovetop flame until warm but still pliable (detailed instructions in notes).: Warmed corn tortillas should smell slightly toasted and feel soft yet sturdy enough to hold fillings. Charring them briefly adds smoky complexity, but overheating leads to brittleness, so aim for pliability rather than crispness to prevent tearing when assembling.
Assemble Street Tacos by layering warmed corn tortillas with carne asada steak, pico de gallo, guacamole, sour cream and cotija. Serve with optional lime wedges and hot sauce.: The finished taco should combine textures and temperatures, with bright pico de gallo cutting through rich guacamole and savory steak. A common assembling error is overfilling tortillas which makes them hard to eat, so balance portions to keep them tidy and enjoyable.
Mash avocados in a medium bowl to desired consistency. Fold in all remaining ingredients. Taste and add additional lime juice or salt to taste.: When you mash the avocados , you will sense a silky, cool texture that contrasts the warm steak, and folding in the aromatics develops a layered flavor. Overworking the avocado can make it overly loose, so stop mashing when you reach a chunky creamy balance.
Serve immediately or cover with plastic wrap (press the plastic so it is touching the top of the guacamole to prevent browning) and refrigerate until ready to serve.: Fresh guacamole should look vibrant and smell herbaceous from the cilantro and lime, and pressing plastic directly on the surface minimizes air contact. A common mistake is letting guacamole sit uncovered which speeds oxidation, resulting in an unappetizing brown layer.