5tablespoonsunsalted butterplus extra for buttering the pan
1cupthinly sliced mushrooms
½ouncedry porcini mushroomsbroken into small pieces
1cupchicken or vegetable brothhomemade or canned
2cupshotbut not boiling, milk (low-fat is fine)
A grating of fresh nutmeg
Salt and freshly ground black, white, or cayenne pepper to taste
12ounceswide noodlessuch as egg noodles
6ouncessharp Cheddar cheeseshredded
1(5-ounce) cans good-quality tunapacked in oil or water, drained
½ to ¾cupfresh peasblanched, or frozen peas, defrosted
3tablespoonschopped fresh parsley
3 to 4tablespoonsdry breadcrumbs
Preheat the oven to 375°F.
In a large, heavy saucepan, melt 4 tablespoons of the butter over medium-low heat. Add the onion and cook until it softens; add the mushrooms and raise the heat slightly to cook them through. Sprinkle the onion and mushrooms with flour, stir for a few minutes to cook out the flavor of the flour, add the dried porcini to the pot, and remove from the heat. Stir in the broth with a wooden spoon and keep stirring to break up any floury lumps and make a smoothish sauce. Stir in the milk and return to the stove.
Cook the sauce over medium-high heat, stirring, for about 7 to 10 minutes, or until it thickens. If there are any floury lumps, whisk the sauce briefly with a wire whisk. Remove from the heat and season with the nutmeg and salt and pepper. Set the sauce aside.
Cook the pasta in a large pot of rapidly boiling salted water until it is just not quite al dente, as it will cook further in the oven, and drain.
In a buttered large shallow pan or baking dish, in layers add a quarter of the sauce, a quarter of the cheese, and a third of the tuna, peas, and noodles. Repeat layers two more times, and then end with a layer of sauce and a layer of cheese. Sprinkle with the parsley and crumbs, then dot with the remaining tablespoon of butter.
Bake until bubbly, lightly browned, and crisped in spots, about 30 minutes. Serve immediately.