In a medium bowl, combine the bacon, onions and mayonnaise.
Place the tomatoes stem side down on cutting board. Cut a thin slice off the bottom. Scoop out the pulp with a melon ball scooper. Invert the tomatoes on a paper towel for 30 minutes to drain.
Fill the tomatoes with the bacon mixture. Refrigerate several hours, or overnight.
Notes
*If you can find one, they do sell small serrated scoopers that are specifically designed for scooping out the pulp of cherry tomatoes. They are a little easier to work with than a melon ball scooper. Check out specialty food or kitchen shops for this cool tool.
How to cook bacon in the oven: Preheat your oven to 400 degrees F. Line a rimmed baking sheet with foil. Lay the bacon on the baking sheet. Don't overlap the bacon or it will all stick together! Bake the bacon for 15 to 20 minutes, until crisp. The time varies depending on how thick your bacon is. Move the bacon slices to paper towels to drain.
If you are preparing this recipe as GLUTEN FREE, just be sure to use brands of mayonnaise and bacon that are known to be GF.