Heat the oil in a large skillet or sauté pan over medium heat.
Add the diced onion and zucchini. Cook until softened and most of the moisture has cooked away. If the pan becomes dry, add 1–2 tablespoons of water.
Add the ground beef. Season with salt, black pepper, and garlic powder. Cook until the beef is browned, then drain or spoon off any excess grease.
Stir in the tomato paste and Worcestershire sauce. Cook for about 1 minute.
Add the diced tomatoes with their liquid, the drained lentils, and the diced bell peppers.
Add Italian seasoning. Simmer until everything is heated through and the filling has thickened slightly. Taste and adjust the seasonings as needed.
To Make Zucchini Boats
Cut zucchini in half lengthwise and scoop out the centers. Season with oil, salt, and pepper, then bake until partially tender.
Fill each zucchini half with the warmed meat filling. Top with mozzarella cheese and return to the oven until the zucchini is tender and the cheese is melted.