1. Preheat oven to 375°. Cut squash in half lengthwise; discard seeds. Bake squash, cut sides down, on a lightly buttered baking sheet 30 to 40 minutes or until flesh is tender when pierced with a fork. Scrape inside of squash to remove spaghetti-like strands. Transfer to a serving platter; toss with butter and 1/2 teaspoon salt. Cover and keep warm.
2. Heat oil in a large deep skillet over medium-high heat. Add mushrooms, next 3 ingredients, and remaining 1/2 teaspoon salt; cook 5 minutes or until mushrooms are tender and liquid evaporates. Stir in wine; bring to a boil. Add spinach and cook, stirring occasionally, 2 minutes or until wilted.
3. Spoon mushroom mixture onto spaghetti squash, and sprinkle with Parmesan cheese.