In a large pot of boiling salted water, cook 1 pound pasta shells until al dente, according to package instructions; drain.
In a large bowl, toss warm pasta with 5 cups diced beefsteak tomatoes (2 pounds), 2 cups diced fresh mozzarella (3/4 pound), 1 halved and thinly sliced small red onion, 1/4 cup extra-virgin olive oil, 1/4 cup chopped fresh flat-leaf parsley, and 1 minced garlic clove. Season with salt and pepper. Serve.