Ingredients

• 2 tablespoons olive oil
  
• 1 ½ cups chopped yellow onion
  
• 1 red bell pepper, chopped
  
• 8 ounces cremini mushrooms, chopped
  
• 2 medium zucchini, chopped into ½-inch pieces
  
• 4 cloves garlic, minced
  
• 48 ounces marinara sauce
  
• 1 teaspoon dried Italian seasoning
  
• Salt and pepper, to taste
  
• 2 pinches crushed red pepper flakes, or to taste
  
• ¼ cup chopped fresh parsley
  
• 5 ounces baby spinach, coarsely chopped
  
• 1 egg
  
• 1 cup ricotta cheese (8 ounces)
  
• 9 oven-ready (no-boil) lasagna noodles
  
• 16 ounces shredded mozzarella cheese (about 4 cups)
  
• ½ cup grated Parmesan cheese
  
• Chopped fresh basil or parsley, optional for serving

Instructions

1. Preheat the oven to 375°F.
2. Cook the vegetables:
   Warm the olive oil in a large skillet or Dutch oven over medium-high heat. Add the onion, bell pepper, mushrooms, and zucchini. Sauté for 7–8 minutes, stirring now and then, until the vegetables soften. Mix in the garlic and cook for another 30 seconds.
3. Build the sauce:
   Pour in the marinara sauce, then season with Italian seasoning, salt, pepper, red pepper flakes, and the parsley. Stir everything together and bring it to a gentle simmer. Lower the heat and let the sauce bubble softly for 10–15 minutes, stirring occasionally. Add the chopped spinach and stir to wilt.
4. Prepare the ricotta layer:
   In a medium bowl, lightly beat the egg with a fork. Add the ricotta and mix until combined. If the mozzarella cheese isn’t shredded yet, do that now.
5. Assemble the lasagna:
   Spread about 1 cup of the sauce into the bottom of a 9×13-inch baking dish.
   
   • Layer 3 lasagna noodles over the sauce.
     
   • Spoon ⅓ of the remaining sauce over the noodles.
     
   • Add half of the ricotta mixture in small dollops.
     
   • Sprinkle ⅓ of the mozzarella and ⅓ of the Parmesan on top.
6. Repeat the layer:
   Add another set of 3 noodles, half of what’s left of the sauce, the rest of the ricotta mixture, and then half of the remaining mozzarella and Parmesan.
7. Finish the top:
   Place the final 3 noodles over the layers. Spread on the last of the sauce and finish with the remaining mozzarella and Parmesan.
8. Bake:
   Cover the dish with foil sprayed on the underside with cooking spray so the cheese doesn’t stick.
   Bake for 35 minutes. (Setting a rimmed baking sheet on the rack below can catch any bubbling sauce.)
   Remove the foil and bake another 10–15 minutes, or until the top is lightly browned and the edges are bubbling.
9. Rest:
   Let the lasagna sit for 15 minutes before cutting into it. This helps the layers firm up for cleaner slices.

• Make Ahead:
  Assemble the lasagna up to 24 hours in advance, cover, and refrigerate. Add about 15 minutes to the covered bake time if starting with a chilled dish.
• To Freeze (Unbaked):
  Wrap tightly and freeze for up to 3 months. Thaw in the refrigerator for 24 hours before baking. Add about 15 minutes to the covered bake time.
• Leftovers:
  Refrigerate for 3–4 days or freeze for up to 3 months. Individual frozen portions reheat especially well. Warm in the microwave or in the oven (covered) until thoroughly heated.
• Half Batch:
  You can prepare half the recipe using a 9×9-inch baking dish.