If using dried lasagna noodles, cook them in generously salted water according to the package directions, stopping about 5 minutes before they reach al dente. Drain and arrange them in a single layer on a lightly oiled baking sheet. Skip this step when using oven-ready noodles.

Heat the olive oil in a large saucepan over medium-high heat. Add the diced onion and minced garlic and cook for 1–2 minutes, until the onion begins to soften.

Add the sliced mushrooms. Cook for 4–5 minutes, stirring occasionally, until the mushrooms have reduced and the onion is translucent. Season well with salt and black pepper.

Sprinkle the flour over the mushroom mixture. Stir thoroughly and cook for about 30 seconds.

Gradually pour in the vegetable stock while stirring continuously. Bring the mixture to a gentle simmer and cook for 8–10 minutes, stirring occasionally, until it begins to thicken.

Stir in the heavy cream. Return to a gentle simmer and cook for another 8–10 minutes, or until the sauce is creamy and lightly thickened.

Remove the saucepan from the heat. Stir in the spinach and about two-thirds of the Parmesan. Continue stirring until the spinach wilts and the cheese melts. Taste and adjust the salt and pepper.

Preheat the oven to 375°F (190°C). Spread a thin layer of mushroom sauce over the bottom of an approximately 8 x 10-inch baking dish.

Add a layer of noodles, followed by mushroom-spinach sauce and mozzarella. Continue layering until the ingredients are used.

Finish your Mushroom Spinach Lasagna with the remaining mozzarella and Parmesan.

Bake for 30–40 minutes, until the cheese is melted, the sauce is bubbling around the edges, and the top is lightly golden.

Allow the lasagna to rest for about 10 minutes before slicing. Garnish with fresh parsley and serve warm.