Preheat and prep. Heat your oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish. Shred the rotisserie chicken into bite-size pieces and set aside.

Sauté the aromatics. Warm olive oil in a skillet over medium heat. Add onion and a pinch of salt. Cook 4–5 minutes until translucent. Stir in garlic, Italian seasoning, and red pepper flakes, and cook 30 seconds until fragrant.

Build the sauce. Pour in marinara and crushed tomatoes if using. Simmer for 5 minutes, then stir in the shredded chicken. Taste and season with salt and pepper. You want a slightly saucy mixture so the noodles cook well.

Mix the cheese layer. In a bowl, combine ricotta, cottage cheese, eggs, 1/4 cup Parmesan, and a pinch of salt and pepper. Stir until smooth. Fold in chopped spinach.

Start layering. Spread a thin layer of sauce on the bottom of the baking dish. Add a layer of noodles, slightly overlapping.

First full layer. Dollop and spread one-third of the ricotta mixture over the noodles. Spoon over a generous layer of the chicken sauce. Sprinkle with about one-third of the mozzarella.

Repeat. Add another noodle layer, then another round of ricotta, sauce, and mozzarella. Repeat once more for three total layers, finishing with noodles, remaining sauce, and the last of the mozzarella. Top with the remaining Parmesan.

Cover and bake. Cover the dish tightly with foil, tenting slightly so it doesn’t stick to the cheese. Bake for 25 minutes.

Uncover and finish. Remove the foil and bake another 15–20 minutes, until the top is bubbly and lightly browned and a knife slides in easily.

Rest and serve. Let the lasagna rest for 10–15 minutes to set. Slice, garnish with basil or parsley if you like, and serve warm.