Preheat the oven according to lasagna noodle package instructions if using dried noodles.

In a large skillet, heat the olive oil over medium heat. Add the diced onion and sauté until translucent.

Add the minced garlic and sliced mushrooms to the skillet, cooking until the mushrooms are softened.

Season the mixture with kosher salt and black pepper to taste.

Stir in the flour and cook for a minute, then gradually add the vegetable stock while stirring to avoid lumps.

Add the heavy cream and bring the mixture to a simmer.

Once thickened, add the fresh spinach and cook until wilted.

In a lasagna dish, layer the ingredients starting with a layer of the mushroom-spinach mixture, followed by a layer of lasagna noodles, and then a sprinkle of parmesan cheese.

Repeat the layers until all ingredients are used, finishing with a layer of noodles topped with mozzarella and the remaining parmesan cheese.

Cover the dish with foil and bake for 30 minutes. Remove the foil and bake for an additional 10 minutes until the cheese is golden and bubbly.

Let the lasagna rest for a few minutes before slicing. Garnish with chopped parsley if desired.