Toast the walnut halves in a dry skillet over medium heat until fragrant and lightly browned, about 3-5 minutes.: As the nuts warm, you will smell a toasty, slightly sweet aroma and see their surface take on a deeper, golden brown color; this signals the oils have released and the flavor has intensified. Keep the skillet at medium so they do not scorch, and stir frequently so each piece browns evenly. Listen for a low crackle when moisture leaves the nuts, and remove them the moment they turn golden to prevent bitterness. If you leave them too long they will burn, which produces an acrid taste that cannot be rescued. Let them cool on a plate before adding to the salad so they stay crisp.
In a large bowl, combine the lettuce, tomatoes, onion, toasted walnuts, and blue cheese crumbles.: When combining, notice the contrast of textures, from the crispness of the romaine lettuce to the creamy patches of blue cheese and the crunchy walnut halves . Toss gently so the leaves do not bruise and the tomatoes do not release all their juices prematurely. A gentle lift and fold motion helps distribute ingredients without mashing the tomatoes. A common mistake is over tossing, which wilts the lettuce and mutes the crisp mouthfeel you want. Aim for an even scatter of components so every portion has a balanced bite.
In a small bowl, whisk together balsamic vinegar, olive oil, Dijon mustard, salt, and pepper. Adjust seasonings to taste.: You will see the vinaigrette come together into a glossy emulsion; the mustard acts as a binder so the oil and vinegar do not separate immediately. Whisk until the dressing slightly thickens and looks cohesive, then taste for balance. The dressing should be bright and tangy with a silky finish, not overly sharp. If it tastes too acidic add a whisper more olive oil , but if it seems flat, add a dot more balsamic vinegar or a pinch of salt. Over seasoning at this stage can make the salad too salty, especially once the blue cheese is mixed in.
Drizzle the dressing over the salad or toss gently to combine. Serve immediately.: After dressing, the salad should glisten with a light sheen, each leaf coated but not soaked. Use a gentle tossing motion so the blue cheese crumbles disperse without breaking down completely. Watch for visual cues like a slight shine on the leaves and the vinaigrette clinging to the tomato skins. If the salad looks watery, you may have used too much dressing, so reserve some and add gradually. Serve immediately to preserve the toasted crunch of the walnut halves and the crispness of the romaine lettuce .