While holding the tomato over a bowl, use a small spoon to scoop out the seeds, juice, and inner tomato pieces. Dice any large inner tomato pieces and reserve, along with the seeds and juice.
Meanwhile, core each tomato to remove the stem. Slice off a ¼ of the top of each tomato. Using a sharp paring knife or a small serrated knife, cut around the inside of the tomato to loosen the seeds and inner tomato pieces.
In a large skillet over medium-high heat, cook ground beef and onion, breaking up beef with a wooden spoon; drain well.