Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish. Wash the bell peppers, slice them in half lengthwise, and carefully remove the core and seeds. Place the pepper halves cut-side up in the prepared baking dish.

Heat olive oil in a large skillet over medium-high heat. Add the ground beef and cook, breaking it up with a spoon, until browned. Drain any excess grease.

Add the diced onion to the skillet with the beef and cook until softened, about 5-7 minutes. Stir in the minced garlic, diced zucchini, and diced mushrooms, cooking for another 3-5 minutes until vegetables are slightly tender.

Stir in the tomato paste, Italian seasoning, smoked paprika, salt, and black pepper. Cook for 1-2 minutes, stirring constantly, to toast the spices.

Remove the skillet from heat. Stir in the breadcrumbs and grated Parmesan cheese until well combined. The mixture should be moist but not overly wet.

In a medium bowl, whisk together the crushed tomatoes, beef broth, garlic powder, and onion powder for the sauce. Pour about half of this sauce into the bottom of the baking dish around the bell peppers.

Carefully spoon the beef and vegetable filling into each bell pepper half, mounding it slightly. Pour the remaining tomato sauce evenly over the stuffed peppers.

Cover the baking dish tightly with aluminum foil. Bake for 30 minutes. Remove the foil, sprinkle additional Parmesan cheese over the tops of the peppers, and bake uncovered for another 10-15 minutes, or until the peppers are tender and the cheese is melted and bubbly.

Let the stuffed bell peppers rest for a few minutes before serving. Serve hot, perhaps with a side salad or a crusty bread (if not strictly avoiding all grains).