Prep the oven and peppers. Preheat your oven to 400°F (200°C). Slice the tops off the bell peppers and remove the seeds and membranes. If they wobble, trim a thin slice off the bottoms to help them stand. Brush the insides with 1 tablespoon olive oil and a pinch of salt.

Pre-bake the peppers. Place peppers upright in a baking dish. Add 1/4 cup water to the pan to create steam. Cover with foil and bake for 12–15 minutes to soften slightly. This helps them cook evenly without collapsing.

Brown the beef. While peppers bake, heat the remaining 1 tablespoon olive oil in a large skillet over medium-high heat. Add ground beef and cook, breaking it up, until browned and no longer pink, about 6–8 minutes. Drain excess fat if needed.

Add aromatics and seasoning. Stir in garlic, Italian seasoning, oregano, red pepper flakes, 1/2 teaspoon salt, and a few grinds of black pepper. Cook 1 minute until fragrant.

Stir in sauce and extras. Reduce heat to medium-low. Add low-carb marinara, diced onion, mushrooms, and olives if using. Simmer 3–4 minutes to thicken slightly. Taste and adjust salt and pepper.

Mix in cheese. Remove skillet from heat. Stir in half the mozzarella and half the Parmesan for a creamy, cheesy filling.

Fill the peppers. Remove the pre-baked peppers from the oven. Spoon the beef mixture into each pepper, packing it in gently. Top with remaining mozzarella and Parmesan. Add pepperoni slices on top if using.

Bake until bubbly. Return to the oven, uncovered, and bake 12–15 minutes, or until cheese is melted and lightly browned.

Finish and serve. Let peppers rest 5 minutes so the filling sets. Garnish with fresh basil or parsley. Serve hot with a simple green salad or roasted veggies.