1. Finely chop the red onion and add to a medium-sized bowl. (You could also use shallots.) Mince the garlic and add to the bowl. Then halve the tomatoes and add to the bowl.
2. Cut the avocados in half and remove the pits. Scoop each half out of the shell and dice. Add the diced avocado to the bowl. Squeeze the lemon half over the top to release its juices. (This will help keep the avocados bright green as well as taste good!)
3. Drizzle with olive oil and season with salt and pepper. Gently toss, being careful not to mush up the avocado. Serve immediately or refrigerate, if not serving right away.