Preheat your oven to 375°F. Slice the tops off the bell peppers and remove the seeds and membranes. Set aside.
Heat a drizzle of olive oil in a large oven-proof skillet over medium heat. Add the chopped onion and cook until soft and translucent, about 4 to 5 minutes.
Add the diced zucchini or squash and cook for another 2 to 3 minutes, until just tender. Stir in the garlic and cook for about 30 seconds.
Add the ground beef to the skillet. Cook until fully browned, breaking it up as it cooks. Drain excess fat if needed.
Stir in 1 cup of the tomato sauce, Italian seasoning, salt, paprika, and lemon juice. Simmer for 3 to 5 minutes, until slightly thickened.
Arrange the peppers in the skillet and spoon the beef mixture evenly into each bell pepper. Top with olive oil.
Cover loosely with foil and bake for 30 minutes. Remove the foil and bake an additional 10 to 15 minutes, until the peppers are soft and the filling is bubbling.
Let the peppers rest for about 5 minutes before serving.