If you're 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.

Heat a large saucepan over medium-high heat. When pan is hot, drizzle in the olive oil.

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

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

Add salt and pepper to taste and stir to combine.

Add flour to saucepan and stir to combine. Let cook for 30 seconds to ensure the flour cooks through.

Add the vegetable stock to the pan gradually, stirring to break up lumps of flour. Bring to a simmer and cook for 8-10 minutes.

Add heavy cream to sauce and stir to combine. Return sauce to a simmer and cook 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 and cheese has melted.

Taste the sauce and adjust seasonings as needed.

Heat oven to 375 degrees F. Spread a thin layer of sauce over the bottom of a medium-sized baking dish.

Layer lasagna noodles, sauce, and mozzarella cheese. Repeat until you run out of ingredients. Top with remaining mozzarella and parmesan.

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

Let lasagna cool for 10 minutes before serving. Top with fresh parsley, cut, and enjoy.