Halve squash crosswise; remove seeds. Place cut sides down in 2-quart baking dish. Add 1/4 C water. Cover with plastic wrap. Microwave on high 13-15 minutes or until squash is tender when pierced with fork; rearrange once for even baking.
In a large skillet cook sausage, mushrooms, bell pepper, onion, and garlic over medium heat until sausage is no longer pink; stir to break up sausage. Drain off fat.
Preheat oven to 350 degrees.
Scrape pulp from squash. Wipe out baking dish; coat with non-stick cooking spray. Spread half of the squash in dish. Add half the sausage mixture and half the olives. Sprinkle with seasoning and 1/8 teaspoon black pepper. Top with half of the spaghetti sauce and cheese. Top with remaining squash, sausage, olives, and spaghetti sauce. Bake for 30 minutes. Sprinkle with remaining cheese. Bake 5 minutes more. Let stand 10 minutes. Sprinkle with parsley and serve.