Gently rub tomatoes under cold running water. Scrub the onion with a clean vegetable brush under cold running water. Rinse the fresh herbs under cold running water.
Thinly slice the onion. Slice the bocconcini cheese.
Place tomatoes, red onion, and bocconcini in a shallow bowl.
Drizzle with olive oil and red wine vinegar. Toss to combine.
Season with salt, pepper, and fresh herbs to taste.