How to Make It
Heat a large, deep skillet. Add the merguez, cover and cook over moderate heat, turning once, until well browned and cooked through, about 15 minutes. Transfer the merguez to a plate. Let cool and thinly slice.
Add 2 tablespoons of olive oil to the skillet. Add the oyster and shiitake mushrooms and season with salt and pepper. Cover and cook over moderate heat, stirring occasionally, until softened and browned, about 15 minutes. Stir in the red onion and the remaining 1 tablespoon of olive oil, cover and cook, stirring, until the onion is softened, about 7 minutes. Remove from the heat and stir in the flour. Set the skillet over moderately high heat and gradually stir in the chicken stock and wine until smooth. Simmer, stirring, until slightly thickened, about 1 minute. Add the merguez and season with salt and pepper.
In a large pot of boiling, salted water, cook the lasagne noodles until just tender, about 3 minutes. Drain the noodles, transfer to a bowl of cold water or let stand for 1 minute. Pat the noodles dry, then toss lightly with olive oil. In a large bowl, blend the ricotta with the pecorino and season with salt and pepper.
Preheat the oven to 375°. In a well-buttered 9-by-13-inch, deep ceramic baking dish, arrange 2 to 3 lasagna noodles in a single layer. Spread half of the mushroom mixture over the noodles. Cover with more noodles and dollop half of the cheese mixture on top. Cover with another layer of noodles, pressing gently to spread the cheese in an even layer. Continue layering with the remaining mushroom mixture, noodles and cheese mixture, ending with a layer of noodles. Spread the softened butter on the top and garnish with the grated Parmigiano-Reggiano cheese.
Cover the lasagna with foil and bake for about 40 minutes, until heated through. Uncover and preheat the broiler. Broil the lasagna about 4 inches from the heat until golden brown and crisp on top, about 2 minutes. Let the lasagna rest for 15 minutes. Garnish with parsley leaves, cut into squares and serve.
Some dried-pasta makers, like Barilla, sell flat lasagna noodles that are shorter and thinner than traditional dried lasagna noodles and work well for delicate lasagnas like this one.