Chop the tomatoes and the onion into small pieces, finely chop or press the garlic clove. Put everything together in a bowl (with any juice from the tomatoes), season with salt and pepper, add basil. Drizzle with oil.
Personally, I use quite a bit of oil, definitely 5 tablespoon on 2 tomatoes, but I love the sauce! A fresh baguette or toasted white bread - very tasty!