1. Preheat your oven to 375 F (190 C). Wash your bell peppers thoroughly. Carefully slice each bell pepper in half lengthwise, from stem to base. Remove the seeds and any white membranes from the inside. Place the pepper halves cut-side up in a baking dish.

2. Heat the olive oil in a large skillet over medium-high heat. Add the ground beef and cook, breaking it up with a spoon, until it’s fully browned and no pink remains. Drain any excess grease from the skillet.

3. Reduce the heat to medium. Add the chopped onion to the skillet with the cooked meat and saute for about 3-5 minutes, until the onion softens and becomes translucent. Stir in the minced garlic and cook for another minute until fragrant, being careful not to burn it.

4. Stir the taco seasoning mix into the meat and onion mixture. Pour in the water or beef broth and add the drained diced tomatoes, if using. Bring the mixture to a simmer and cook for 5-7 minutes, stirring occasionally, until the liquid has mostly evaporated and the mixture has thickened.

5. Carefully spoon the prepared taco meat filling into each bell pepper half, mounding it slightly.

6. In a small bowl, combine the shredded cheddar and Monterey Jack cheeses. Sprinkle a generous amount of the cheese mixture over the top of each stuffed bell pepper. Place the baking dish in the preheated oven.

7. Bake for 25-30 minutes, or until the bell peppers are tender-crisp and the cheese is melted, bubbly, and lightly golden brown.

8. Carefully remove the baking dish from the oven. If desired, garnish with fresh chopped cilantro. Serve hot with your favorite low-carb toppings like sour cream, plain Greek yogurt, or sliced avocado.