Heat a generous splash of olive oil in a large skillet over medium heat. Add chopped onions and sauté until translucent, about 5 minutes.

Add minced garlic and cook for an additional minute until fragrant.

Add your sliced mixed mushrooms and sauté until browned and moisture is released, around 8–10 minutes.

Sprinkle in dried thyme, oregano, salt, and pepper, then pour in white wine if desired. Cook for 2–3 minutes until liquid evaporates.

Stir in halved grape tomatoes and cook until softened, about 2–3 minutes. Set aside.

In a medium saucepan, melt butter over medium heat. Whisk in flour and cook for 2–3 minutes until golden.

Gradually add warmed milk while whisking until thickened, about 5–7 minutes. Season with nutmeg, salt, and pepper.

Preheat oven to 375°F (190°C). Cook lasagna noodles in boiling salted water until al dente, about 8–10 minutes. Drain and lay flat.

In a baking dish, spread a thin layer of Bechamel sauce. Layer noodles, mushroom-tomato mixture, spinach if using, Bechamel, and cheeses. Repeat until all ingredients are used.

Cover with foil and bake for 25 minutes. Remove foil and bake for an additional 20 minutes until top is golden.

Let lasagna rest for about 10 minutes, then serve with fresh basil if desired.