In a medium bowl, combine the ricotta cheese, egg whites, Parmesan cheese, ½ cup mozzarella, salt, and pepper.
15 ounces ricotta cheese, 2 egg whites, ½ cup Parmesan cheese, 1 ½ cups shredded mozzarella, ¼ teaspoon salt, ½ teaspoon pepper
Spray the inside of the slow cooker with cooking spray. Cover the bottom with about half of the ziti noodles.
12-16 ounces ziti noodles
Pour about 2 cups of spaghetti sauce over the top and spread evenly with a spoon. Then distribute about half of the ricotta mixture on top of the sauce and spread evenly. Place the mushrooms on top of the ricotta, and then the spinach.
Place remaining noodles on top, then spread another 2 cups of sauce, then spread the remaining ricotta cheese mixture.
Pour the remaining sauce on top and spread evenly. Cook on low for about 4 hours, but no more than 5.
About 5 minutes before serving, remove lid and sprinkle the remaining mozzarella cheese over top. Replace the lid and let the cheese melt before serving.
Notes
If you want to use frozen spinach, make sure to thaw it first and squeeze out the excess liquid.
Include cooked ground beef, turkey, sausage, or meatballs if desired. Layer it between the pasta and sauce for even distribution.
If you aren't a fan of mushrooms, you can swap them out for for diced zucchini, bell peppers, or even eggplant.
Add a teaspoon of Italian seasoning or crushed red pepper flakes to the ricotta mixture for added flavor.
Substitute mozzarella with provolone or Fontina for a creamier texture.
To freeze leftovers, allow the ziti to cool to room temperature before freezing. If desired, divide into single-serving airtight containers or freezer bags. When ready to use it, thaw overnight in the fridge and then reheat in the oven at 350°F (covered with foil) or in the microwave until warmed through.
Additional Serving Suggestions
For a complete meal, serve with garlic bread or breadsticks and pair with a side salad with vinaigrette to balance the richness of the ziti.
Top individual servings with fresh Parmesan, a dollop of ricotta, or a sprinkle of fresh basil or parsley.