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.

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.

Add the vegetable stock to the pan a little at a time, stirring as you go to break up any lumps of flour. Bring mixture to a simmer and cook, stirring occasionally, for 8-10 minutes until sauce has begun to thicken.

Add heavy cream to sauce and stir to combine. Return sauce to a simmer and cook, stirring occasionally, for an additional 8-10 minutes until sauce has thickened to your desired consistency.

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 into the sauce.

Taste the sauce and adjust seasoning as needed.

Heat oven to 375 degrees F.

Spread a thin layer of sauce over the bottom of a medium-sized baking dish. Put down a layer of lasagna noodles, followed by a few ladlefuls of sauce, followed by a few slices of mozzarella cheese. Repeat until you run out of noodles and sauce. Top the final layer with any remaining mozzarella cheese and the rest of your parmesan.

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

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