Lasagne alla Boscaiola (Italian White Lasagna with Sausage, Mushrooms and Peas)

"Alla Boscaiola" translates to something like "the woods" so this hearty lasagna is stuffed with mushrooms (any variety will work but I chose cremini), fennel pork sausage, and green peas (don't skip them, they add just the right amount of sweetness to this dish). Because it's a white lasagna the noodles and filling are layered with a creamy white sauce called besciamella (made with a butter and flour roux, milk, and a pinch of nutmeg). And there's no mozzarella or ricotta, just generous amount of Parmesan and/or Pecorino (I recommend using both).

Besciamella:
¼ cup butter
¼ cup all-purpose flour, unbleached
3½ cups whole milk, warmed
¼ tsp. sea salt
pinch ground nutmeg

Lasagna:
1 lb fennel pork sausage
2 Tbsp. extra-virgin olive oil
½ white onion, diced
8 oz cremini or mixed mushrooms, cleaned and sliced
2 Tbsp. fresh flat-leaf parsley, chopped
¼ tsp. dried oregano
pinch crushed red pepper flakes (optional)
1 Tbsp. white wine vinegar
¼ cup water
1½ cups frozen peas
½ tsp. sea salt
¼ tsp. white pepper, freshly ground
Barilla no-bake lasagna noodles (~1 box, I used 12 sheets total, 3 for each layer)
1¼-1½ cups Parmigiano Reggiano and/or Pecorino Romano, freshly grated

Make the Besciamella:
Melt the butter in a large pan over medium heat (being careful not to brown it). Add the flour and cook for 1 minute, whisking constantly. Slowly pour in the milk, a little bit at a time, while whisking constantly to prevent lumps. Once all of the milk is added, add the nutmeg and salt and bring the mixture to a simmer and cook, still whisking frequently, until thickened. (The besciamella will be thick enough to coat the back of a spoon.) Remove from the heat and set aside. (It can be prepared up to several days ahead of time and refrigerated until ready to use.)

Brown the sausage in a large sauté pan over medium heat, breaking it up into small pieces as it cooks. Once it is just cooked through, drain the grease and set the sausage aside on a plate.

Return the pan to the heat and add the olive oil, onion, and mushrooms. Cook, stirring occasionally, for about 5 minutes until tender. Stir in the parsley, oregano, vinegar, water, and peas. Return the sausage to the pan and bring the mixture to a simmer, then cook for about 5 minutes to warm the peas. Season with the salt and freshly ground black pepper to taste.

Assemble the Lasagna:
Spread a thin layer of besciamella on the bottom of a lightly greased 9x13" baking dish. Then place a single layer of noodles over the sauce. Top with another thin layer of besciamella, ¼ of the filling, and ¼ of the cheese. Repeat for the remaining 3 layers (you will have 4 layers in all) adding noodles, sauce, filling, and cheese. The top layer will end with any remaining besciamella and cheese.

Make-Ahead: Lasagna can be assembled 1-2 days ahead and refrigerated until ready to bake or frozen and baked at a later time.

Preheat oven to 350°F (180°C/160°C fan, Gas Mark 4).

Bake for approximately 45 minutes (possibly shorter if your lasagna is already warm, or longer if your lasagna has been refrigerated/frozen before baking) until golden brown and bubbling.

Let rest 15 minutes before serving.