Step 1

Position rack in upper third of oven; preheat to 450 degrees F.

Step 2

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 degrees F until tender, 40 to 50 minutes.)

Step 3

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, 1/4 teaspoon pepper, crushed red pepper and 1/8 teaspoon salt. Cook until heated through and the flavors have blended, 1 to 2 minutes. Remove from heat and cover.

Step 4

Use a fork to scrape the squash from the shells into a large bowl. Stir in Parmesan, the remaining 1/4 teaspoon pepper and the remaining 1/8 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 1/4 cup mozzarella into each shell. Dollop 1/4 cup ricotta over the mozzarella. Repeat with the remaining squash mixture, tomato sauce and mozzarella.

Step 5

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.