Preheat oven to 400°F. Cut the zucchini in half lengthwise and scoop out the flesh, leaving about 1/4 inch. Chop the scooped flesh and set aside.

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

Brown the sausage in a skillet over medium heat until cooked through. Remove from heat.

Heat a tablespoon of olive oil in a pan. Add finely diced onion, minced garlic, and diced red bell pepper. Cook for 2-3 minutes until translucent.

Add chopped zucchini flesh to the skillet, season with salt and pepper, and cook for another 2-3 minutes.

Combine the cooked sausage with the sautéed vegetables and cook a few more minutes to blend flavors.

Fill each hollowed zucchini with the sausage mixture, pressing firmly. Top with marinara sauce, shredded mozzarella, and Parmesan cheese.

Place stuffed zucchinis in a baking dish and cover with foil. Bake for 35 minutes until cheese is melted and zucchini is tender. Remove foil in the last 10 minutes to brown the cheese slightly.