In a small bowl, massage onions with 1/2 teaspoon salt. Let sit for a few minutes (or refrigerate for up to 2-3 hours) while you prepare the rest of the salad.
Place tomatoes and peppers in a medium bowl. Add onion mixture, vinegar, and oil. Stir to combine. Taste to adjust seasoning, adding additional salt if needed so the flavors pop vibrantly.