Cook the pasta in salted water for about 10 minutes, strain. Preheat the oven to 220 degrees C.
In a large bowl, carefully mix the noodles, onions, 1 cup cheese, peas, tuna, stock and mushrooms. Put everything in the roasting pan, and on top of the potato chips crumbs and the remaining glass of cheese.
Bake for 15 to 20 minutes in a preheated oven.