Cut zucchini in half lengthwise. Using a spoon, scrape out the inside of the zucchini leaving a ½-inch shell.*
Place zucchini in a large baking dish, cut-side up. Brush with 1 tablespoon of olive oil and season with salt and pepper to taste. Bake for 15 minutes.
Meanwhile, in a large skillet, brown Italian sausage and onion over medium-high heat. Drain any fat. Stir in the garlic and red pepper and cook for an additional 2-3 minutes or until softened.
Stir in diced tomatoes and Italian seasoning. Taste, and season with additional salt and pepper to taste.
Fill the baked zucchini with the sausage mixture. Top with the mozzarella and parmesan cheese. Bake for an additional 10-15 minutes until the zucchini is soft and the cheese is bubbly.
Notes
*Save the scooped-out zucchini flesh, remove any large or tough seeds, and add it to the sauce.
If adding extra veggies to the sauce, cook the sauce for extra time to evaporate some moisture. This keeps the sauce filling from getting too watery when baked.
Prep the zucchini boats and sauce in advance and assemble before baking for easy weeknight cooking.