Preheat your oven to 375 degrees F (190 degrees C). Lightly grease a 9x13-inch baking dish with cooking spray or olive oil.

In a large skillet over medium-high heat, cook the ground beef or Italian sausage with the diced onion, breaking up the meat as it cooks. Cook for 7-8 minutes until the meat is fully browned and the onions are soft. Drain excess grease well.

Add the diced bell pepper and sliced mushrooms to the same skillet over medium heat. Cook for 4-5 minutes, stirring occasionally, until the peppers are tender-crisp and the mushrooms have released and cooked off their moisture.

Stir in the garlic powder, dried oregano, salt, and black pepper. Toss to coat the meat and vegetables evenly, then remove the skillet from the heat.

Transfer the meat and vegetable mixture to the prepared baking dish and spread it into an even layer across the bottom.

Pour the low-carb marinara sauce evenly over the meat layer. Spread toward the edges so everything is coated, stirring gently to let the sauce work into the meat if needed.

Scatter the shredded mozzarella and Parmesan evenly across the top. Lay the pepperoni slices over the cheese, covering as much surface area as desired.

Bake uncovered for 20-25 minutes, until the cheese is fully melted, bubbling at the edges, and starting to turn golden brown on top with crispy pepperoni edges. If the cheese browns too quickly, lay a sheet of foil loosely over the dish and continue baking.

Remove from the oven and let the casserole rest for 5 minutes before slicing and serving. This allows the sauce to settle for cleaner servings.