1 2½- to 3-pound spaghetti squash, halved lengthwise and seeded
¼ cup water
2 tablespoons extra-virgin olive oil
1 medium onion, chopped
4 cloves garlic, minced
10 ounces mushrooms, sliced
2 cups crushed tomatoes
1 teaspoon Italian seasoning
½ teaspoon ground pepper, divided
¼ teaspoon crushed red pepper
¼ teaspoon salt, divided
¼ cup grated Parmesan cheese
1 cup shredded part-skim mozzarella cheese, divided
½ cup part-skim ricotta cheese
Position rack in upper third of oven; preheat to 450°.
Place squash cut-side down in a microwave-safe dish and add water. Microwave, uncovered, on High until the flesh is tender, 10 to 12 minutes. (Alternatively, place squash cut-side down on a large rimmed baking sheet. Bake at 400°F until tender, 40 to 50 minutes.)
Meanwhile, heat oil in a large skillet over medium heat. Add onion and garlic; cook, stirring, until starting to soften, 3 to 4 minutes. Add mushrooms and cook, stirring, until the vegetables are tender and starting to brown, about 5 minutes more. Add tomatoes, Italian seasoning, ¼ teaspoon pepper, crushed red pepper and ⅛ teaspoon salt. Cook until heated through and the flavors have blended, 1 to 2 minutes. Remove from heat and cover.
Use a fork to scrape the squash from the shells into a large bowl. Stir in Parmesan, the remaining ¼ teaspoon pepper and the remaining ⅛ teaspoon salt. Place the shells cut-side up on a large rimmed baking sheet. Spoon one-fourth of the squash-Parmesan mixture into each shell. Layer one-fourth of the tomato mixture on top, then sprinkle ¼ cup mozzarella into each shell. Dollop ¼ cup ricotta over the mozzarella. Repeat with the remaining squash mixture, tomato sauce and mozzarella.
Bake the squash lasagnas for 15 minutes. Turn the broiler to high and broil, watching carefully, until the cheese starts to brown, 1 to 2 minutes. Cut each lasagna in half to serve.