Preheat and prep. Heat your oven to 375°F (190°C).
Lightly oil a 9x13-inch baking dish. If using regular noodles, cook them until very al dente, then drain and lay on parchment to prevent sticking.
Sauté the aromatics. In a large skillet, warm olive oil over medium heat. Add onion and cook until translucent, about 5 minutes.
Stir in garlic for 30 seconds until fragrant.
Add veggies (optional). If using mushrooms, add them now and sauté until they release moisture and brown slightly. Stir in spinach and cook just until wilted. Season with a pinch of salt and pepper.
Combine with chicken. Add the cooked chicken to the skillet.
Pour in 1 cup of the marinara and toss until everything is lightly coated and warmed. Stir in red pepper flakes and half the chopped basil. Remove from heat.
Make a quick white sauce. In a medium saucepan, melt butter over medium heat.
Whisk in flour and cook 1 minute, stirring constantly. Slowly whisk in milk until smooth. Simmer 3–4 minutes until slightly thickened.
Off the heat, whisk in 1/2 cup Parmesan and a pinch of salt and pepper.
Mix the ricotta layer. In a bowl, combine ricotta, egg, remaining 1/2 cup Parmesan, a pinch of salt, black pepper, and the rest of the basil (or Italian seasoning). Stir until smooth.
Layer the lasagna. Spread a thin layer of marinara (about 1/2 cup) over the bottom of the dish. Add a layer of noodles.
Build Layer 1. Spread 1/3 of the ricotta mixture over the noodles.
Spoon 1/3 of the chicken mixture on top. Drizzle 1/2 cup white sauce. Sprinkle 1 cup mozzarella.
Add a thin layer of marinara.
Repeat layers. Add another layer of noodles and repeat the ricotta, chicken, white sauce, mozzarella, and marinara sequence two more times. Finish with a final layer of noodles, a generous smear of marinara, remaining white sauce, and a final sprinkle of mozzarella.
Cover and bake. Tent the dish with foil (lightly oil the underside so cheese doesn’t stick). Bake for 25 minutes.
Uncover and finish. Remove the foil and bake another 15–20 minutes, until bubbling and lightly golden around the edges.
If you like a deeper top color, broil for 1–2 minutes, watching closely.
Rest and serve. Let the lasagna rest for at least 15 minutes so it sets. Sprinkle with chopped parsley before slicing.