Ingredients:
1 box pasta
1 cup chopped onion, plus olive oil for sautéing
1, 8-ounce package sliced mushrooms
2, 10-ounce cans cream of mushroom soup
1/4 cup pasta water
4 cups shredded Gruyère

Directions:
Preheat the oven to 350. Cook pasta according to package instructions. Meanwhile, in a large pan, add a splash of oil, then add onions and mushrooms. Sauté on medium-low heat, 3 minutes or until veggies are soft. Then, add cream of mushroom soup and pasta water. When pasta is ready, drain and add to mushroom and onion mixture. Transfer the mixture to a casserole dish, then top with Gruyère cheese. Bake 20 minutes.