If using dried lasagna noodles, cook them according to package directions in very salty water until 5 minutes less than al dente. Drain and set aside.

Lay the noodles out in a single layer on a lightly oiled baking sheet to keep them from sticking.

If using ready-to-bake noodles, skip this step.

Heat a large saucepan over medium-high heat. Drizzle in the olive oil.

Add onion and garlic to the pan and stir to combine. Cook for 1-2 minutes until the onion has begun to soften.

Add sliced mushrooms to the pan and cook, stirring occasionally, for 4-5 minutes until the onion is translucent and mushrooms have reduced in size.

Add salt and pepper to taste and stir to combine.

Add flour to the saucepan and stir. Let cook for 30 seconds.

Gradually add vegetable stock, stirring to break up any lumps of flour. Bring to a simmer and cook for 8-10 minutes until sauce has thickened.

Add heavy cream to the sauce and stir to combine. Simmer for an additional 8-10 minutes.

Turn off the heat, then add spinach and ⅔ of the parmesan cheese to the sauce. Stir until spinach has wilted.

Taste the sauce and adjust seasonings as necessary.

Preheat oven to 375 degrees F.

Spread a thin layer of sauce over the bottom of a baking dish.

Layer lasagna noodles, sauce, and mozzarella cheese until ingredients are exhausted.

Top the final layer with remaining mozzarella and parmesan.

Bake lasagna for 30-40 minutes until cheese is melted and edges are lightly browned.

Let cool for 10 minutes before serving. Garnish with fresh parsley.