Bring a large pot of water to boil.

Preheat oven to 400°F (200°C).

Cut zucchini in half lengthwise and using a spoon or melon baller, scoop out flesh, leaving 0.25 inch thick walls. Chop the scooped out flesh of the zucchini into small pieces and set aside.

Drop zucchini halves into boiling water and cook for 1 minute. Remove from water and drain well.

Place 0.25 cup of sauce in the bottom of a 9 x 12 inch baking dish, and arrange zucchini halves cut side up.

In a large skillet, brown sausage, breaking it up as it cooks into smaller pieces until browned; set aside.

Heat olive oil in the same skillet and add onion, garlic, and bell pepper. Cook on medium-low heat for about 2-3 minutes, until onions are translucent. Add chopped zucchini, season with salt and pepper, and cook for about 2-3 minutes.

Combine the cooked vegetable mixture with the browned sausage and cook for a few more minutes.

Using a spoon, generously fill each hollowed zucchini half with about 0.33 cup of the cooked sausage mixture, pressing firmly.

Top each stuffed zucchini with 2 tablespoons of marinara sauce, 1 tablespoon of shredded mozzarella cheese, and 1 teaspoon of Parmesan cheese.

Cover the baking dish with foil and bake for 35 minutes, or until the cheese is melted and bubbly, and the zucchini is cooked through and tender.