Combine olive oil, vinegar, shallots, mustard, salt, and a few grinds pepper in a small bowl (or jar).
Whisk (or shake, if using a jar) ingredients together until dressing looks thick and creamy.
Season to taste with additional salt and pepper, if desired.
Prepare Salad:
Place sliced red onion in a fine mesh strainer, then rinse well for cold water. Blot dry with paper towels.
Combine onions, tomatoes, basil, and ¼ cup dressing (you will have extra) in a large bowl. Mix well.
Season salad to taste with additional salt and pepper, and add more dressing, if needed.
Serve salad immediately. Enjoy!
Notes
I used two extra-large heirloom tomatoes for this recipe (1 pound 10.5 ounces / 751 grams total). If your tomatoes aren't quite as large, just use more tomatoes. This recipe is extremely flexible, so the exact measurements aren't that vital. You can use less dressing if you have fewer tomatoes, or more dressing if you have extra tomatoes, for example. Almost any kind of tomato (from cherry to beefsteak) works here, just make sure they are the freshest and tastiest tomatoes you can find.Check out the information before this recipe card for helpful tips, recipe variations, serving suggestions, and more!