Remove any stems from the tomatoes. Rinse the tomatoes and pat them dry. Cut a small circle around the core to remove it. Cut each tomato in half, then slice those halves into wedges about 1-inch thick. Place the cut tomatoes in a medium serving bowl.
Peel a red onion and cut it in half. Cut one of the halves in half again. Chop that wedge of onion until you have ⅓ cup of chopped onions and add them to the bowl of tomatoes. (Rrefrigerate the remaining onion for another use.)
Lightly rinse the basil leaves and pat them dry with paper towels. Use kitchen scissors or a knife to cut them into small pieces and add them to the bowl of tomatoes.
Drizzle on the olive oil and vinegar. Sprinkle on the garlic powder, salt and pepper. Toss the tomato salad with two spoons until it's combined.
Serve at room temperature or chilled. Cover and refrigerate any leftovers for up to three days, but it's best to serve it the day you make it. The tomatoes will start getting soft after the first day. Stir leftover tomato salad before eating it.
Notes
Variations: You can omit the onions if you prefer. You also can add black olives, celery, cucumbers or mozzarella cheese. You can use fresh mozzarella or regular.