In a medium saute pan, heat 3 tablespoons of the butter, add the onion and mushrooms, and saute until they soften and the onions release their juices. Add salt and pepper, to taste, and set aside.
Preheat the oven to 375 degrees F.
In a large bowl, combine the mascarpone, Emmentaler, Parmigiano-Reggiano, eggs, and nutmeg, and stir well. Season with salt and pepper and stir in the onions and mushrooms.
In a saute pan, melt the remaining butter and toss in the bread cubes, cooking over high heat until they are crisp.
Place the squash in a baking dish and, using a total of a third of the cheese mixture, stuff the bottom of each of the squash. Using a total of half of the bread cubes, place them in even quantities into each of the squash. Top the bread cube layer with more of the cheese mixture, then the remaining bread cubes, and the remaining cheese. Replace the top on the pumpkin and roast 1 1/2 hours in the oven, until the flesh is very soft. Remove from the oven, let cool for a few minutes and scoop out the cheese and flesh to serve.
Recipe copyright 2001, Mario Batali. All Rights Reserved.