If you're using dried lasagna noodles, cook them in a large pot of well-salted boiling water until about 5 minutes shy of al dente, according to package instructions. Once cooked, drain the noodles and lay them out in a single layer on a lightly oiled baking sheet to prevent them from sticking together—this makes them much easier to handle during assembly. If you're using oven-ready (no-boil) lasagna noodles, you can skip this step entirely. See recipe notes for additional guidance.

Place a large saucepan over medium-high heat. Once the pan is hot, add the olive oil. Stir in the diced onion and minced garlic, cooking for 1–2 minutes until the onion begins to soften. Add the sliced mushrooms and cook for another 4–5 minutes, stirring occasionally, until the onions turn translucent and the mushrooms shrink and release their moisture. Season the mixture generously with kosher salt and black pepper—don’t hold back here, as the salt is key to deepening the flavor. (You can add up to 1 teaspoon of kosher salt at this stage, depending on taste.) Sprinkle the flour over the mushroom mixture and stir well to combine, letting it cook for about 30 seconds to eliminate the raw flour taste.

Slowly pour in the vegetable stock, stirring constantly to break up any lumps of flour. Bring the mixture to a gentle simmer and cook, stirring now and then, for 8–10 minutes until it begins to thicken slightly. Stir in the heavy cream and return to a simmer, cooking for another 8–10 minutes until the sauce thickens further. The final consistency should be rich but still pourable—not quite as thick as a béchamel or cheese sauce, since it will continue to thicken as it cools and when baked with the lasagna noodles. Once the sauce is done, remove the pan from heat. Add the fresh spinach and ⅔ of the shredded Parmesan cheese, stirring until the spinach wilts and the cheese melts into the sauce.

Give it a taste test—this is your chance to adjust the seasoning. Most likely, you'll want to add an extra ½ teaspoon of kosher salt or more, depending on your preference.

Preheat your oven to 375°F (if you're baking it right away—skip preheating if you plan to freeze or refrigerate it for later). In a medium-sized baking dish (an 8x10-inch dish works well), spread a thin layer of the sauce across the bottom to prevent sticking. Lay down your first layer of lasagna noodles, followed by a few generous ladlefuls of the sauce, then a layer of mozzarella slices. Repeat these layers—noodles, sauce, mozzarella—until all ingredients are used up. Finish with a final layer of mozzarella and the remaining Parmesan cheese sprinkled over the top.

Bake for 30–40 minutes until the cheese is fully melted and bubbly and the edges are golden brown. If you're prepping ahead or freezing, skip baking for now and refer to storage instructions in the recipe notes. After baking, let the lasagna rest for 10 minutes before slicing. Garnish with chopped fresh parsley if desired, serve, and enjoy!