Slice the onion in half, then thinly slice the halves (either direction is fine, just different looks).
Core the tomatoes and cut them into 1-inch pieces.
In a large bowl, toss the onions and tomatoes with the vinaigrette. Serve at toom temperature.
If your tomatoes aren’t perfectly ripe… just don’t make this. Make tomato sauce, or if tomatoes are not in season, just wait until they are. I’m sorry to be the bearer of this information, if you are not a ripe tomato devotee, but it’s just true. Also, never put your tomatoes in the fridge, unless they are threatening to turn to mush, and you are planning to cook them anyway. Cold changes the texture of tomatoes, making them mealy, and saps their flavor.