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

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 onion has begun to soften.

Add sliced mushrooms 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 vegetable stock to the pan a little at a time, stirring to break up lumps of flour. Simmer for 8-10 minutes until sauce has thickened.

Add heavy cream to sauce and stir to combine. Simmer for an additional 8-10 minutes until desired consistency is reached.

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

Taste the sauce and add extra kosher salt or pepper as needed.

Preheat 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 repeatedly until ingredients are used up.

Top the final layer with mozzarella cheese and remaining parmesan.

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

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