Heat oven to 375°F. To prepare the zucchini, cut in half lengthwise, then scoop out the seeds and part of the flesh in the center to create a hollowed-out section for the stuffing, leaving about a 1/4" edge. A small teaspoon with a narrow or pointed end works well for this. Save and freeze the part removed to add to soups or quick breads at a later time. Salt and pepper the zucchini and set aside.
Brown the sausage in a skillet with the red pepper flakes, breaking up the sausage into small crumbles as it browns. Once done, drain on paper towels and set aside.
In the same skillet, cook the onions in the fat from the sausage over medium heat until translucent, about 5 minutes (add olive oil if the skillet is too dry). Add the garlic, stir and cook until fragrant, about 2 minutes. Add the olive oil, green pepper, mushrooms, and oregano in with the onions and garlic; cook over medium heat until mushrooms release their liquid and wilt. Remove skillet from heat and allow vegetables to cool.
In a large bowl, mix the sausage, mushroom mixture, tomato, olives, basil, and Parmesan until combined. Gently stir-in the mozzarella cheese.
Pat zucchini dry with paper towels if any liquid has collected in the centers. Spray the bottom of a large casserole dish with cooking spray. Place the zucchini in an oven-safe baking dish and spoon the pizza sauce into the bottom of the zucchini. Top the sauce with the stuffing, mounding it and gently pressing down so it will hold together. Sprinkle with parmesan and drizzle with olive oil.
Bake at 375°F for 30 minutes, or until cheese is melted and browned in spots. Note: The size of the zucchini will dictate cooking time, so test with a fork to determine doneness. If a fork pierces the zucchini with only light resistance, it is crisp-tender and ready to remove from the oven. If a softer texture is preferred, bake another 5-10 minutes.
Notes
Zucchini vary in size, so be flexible with the stuffing. This recipe will fill zucchini 12-inches long and approximately 3 inches in diameter.
To make this gluten-free, use gluten-free sausage.
To make ahead: the stuffing ingredients can be prepared and refrigerated for up to 3 days, except for the tomatoes and basil.
Substitute 1/2 teaspoon of dried basil for fresh basil, if desired.