In a large skillet over medium-high heat, add ground beef and onion. Cook, until browned, breaking up beef with a wooden spoon, about 5 minutes. Drain well.
Meanwhile, core each tomato to remove the stem. Slice off 1/4-inch 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.
While holding the tomato over a bowl, use a small spoon to scoop out the seeds and inner tomato flesh. Dice any large inner tomato pieces and reserve, along with the seeds and juice.
Add tomato sauce, cooked rice, taco seasoning, and any reserved tomato pieces and juice to the skillet. Stir to combine and continue to cook until juices begin to thicken, about 10 minutes. Season to taste with salt and pepper.
Remove skillet from heat and cool filling slightly. Working with one tomato at a time, spoon beef filling into a tomato and mound carefully on top. Repeat with remaining tomatoes.

Notes

Ground beef: Also delicious with ground turkey or chicken.
Tomatoes: Only make this recipe with tomatoes that you'd want to eat raw. Leave the bland, pithy, store-bought tomatoes out of it. Those smaller, on-the-vine Campari tomatoes might be good candidates.
Rice: Any kind of pre-cooked or leftover rice you have: brown rice, leftover Cilantro Lime Rice, long-grain rice, or even quinoa.
Taco seasoning: I make my own taco seasoning blend to have on hand when the urge for tacos hits unexpectedly, but you can use the little packet from the store, too.
Toppings: Try scallions, avocado or guacamole, shredded cheese, sour cream, Fritos, or crumbled tortilla chips.
Size matters: Depending on the size and shape of the tomatoes you use, you may have to scale the recipe up to accommodate them and make a double batch. Luckily, that's easy to do and leftovers are always welcome.
Make ahead: The filling can be made ahead of time and rewarmed in a skillet or microwave. Stuff the tomatoes right before you plan to eat.