PREP: 35 minutesBAKE: 35 minutesSTAND: 10 minutesOVEN: 350°F (180°C) / 175°CMAKES: 6 servings
Prep & Cook the Squash: Preheat your oven to 350°F / 175°C. Carefully halve the spaghetti squash crosswise and scoop out the seeds.
Place the squash halves, cut-sides down, in a microwave-safe 2-quart rectangular baking dish.
Add the water, cover with vented plastic wrap, and microwave on high for 13-15 minutes, or until tender when pierced with a fork.
Drain any excess water and let it cool slightly.
Sauté the Veggies: While the squash cooks, heat the olive oil in a large skillet over medium heat.
Add the mushrooms, bell pepper, onion, and garlic.
Cook for about 5 minutes, stirring occasionally, until the vegetables are tender and fragrant.
Create the Noodles & Assemble Layer One: Once the squash is cool enough to handle, use two forks to scrape and shred the pulp into spaghetti-like strands (you should get about 3 cups / 720 ml). Wipe out the baking dish and coat it with nonstick cooking spray.
Spread half of the squash strands in the bottom of the dish.
Top with half of the sautéed mushroom mixture, then sprinkle with the Italian seasoning and black pepper.
Pour half of the pasta sauce over the top and sprinkle with half of the mozzarella cheese.
Assemble Layer Two & First Bake: Create a second layer with the remaining squash, mushroom mixture, and pasta sauce.
Bake, uncovered, for 30 minutes.
Final Bake & Rest: Sprinkle the remaining mozzarella cheese over the top and bake for about 5 more minutes, until the cheese is bubbly and melted.
Let the casserole stand for 10 minutes before serving—this helps it set up!
Garnish with fresh parsley and enjoy.