Step 1

Heat the olive oil in large nonstick skillet over medium-high heat. Once hot, add the mushrooms and onions, and sauté until they're beginning to brown and soften, about 6 minutes. Add the bell pepper and cook for 3 minutes, then add the garlic. Add the spinach a few handfuls at a time, stirring and cooking until spinach wilts and the garlic smells fragrant, 1 to 2 additional minutes. Remove from the heat and set aside.

Step 2

In a medium bowl, combine the ricotta, egg, salt, and pepper, until combined. Stir in 1/2 cup mozzarella and Parmesan.

Step 3

Generously coat the inside of a 4-quart or larger slow cooker with cooking spray. Spread a thin layer of sauce on the bottom, then top with half of the pasta noodles. Pour 2 cups sauce over the top and spread it into an even layer with the back of a spoon or spatula. Dollop half of the ricotta mixture on top, then sprinkle with half of the sautéed vegetables.

Step 4

Repeat the layers: remaining noodles, 2 cups of sauce, remaining ricotta, and remaining vegetables. Pour the remaining sauce on top and spread evenly. Cover and cook on low for 3 to 4 hours or on high for 1 to 2 hours. Check at the 3 (on low) or 1 (on high) hour mark to ensure the pasta does not over cook. To check for doneness, use a fork to pull a "test" noodle from the center of the slow cooker. Once it is tender, the pasta is done.

Step 5

Sprinkle the top with the remaining 1 cup mozzarella cheese. Cover, turn the heat to high, and cook until the cheese is melted, 5 to 10 minutes. Serve warm, topped with chopped fresh basil or parsley.