Combine tomatoes, onion, mushrooms, peppers, pepperoni, mozzarella, basil and pasta in a big bowl. Whisk garlic salt, oregano or Italian seasoning, tomato paste and vinegar together. Stream in extra-virgin olive oil while continuing to whisk dressing. When oil is incorporated, pour dressing over pasta salad, add a few grinds of black pepper to the bowl, then toss salad to coat evenly. Adjust your seasonings and serve salad. Leftovers make a great lunch or snack the next day!