Spray a 9x13inch casserole dish with non-stick spray and preheat the oven to 375 degrees F.
Melt one tablespoon of butter in one tablespoon of oil in a skillet over medium-high heat. Once hot, add mushrooms and cook for 10 minutes, stirring often until well browned. Remove to a large bowl.
Add remaining butter and oil to the pan and reduce heat to medium and add the chopped onions.
Cook onions for 5 minutes or until they start to get soft, then add garlic and cook for one minute. Add to large bowl with the mushrooms.
Add the cut-up pot roast, along with the parsley and Gruyere cheese, to the large bowl, then toss to combine.
Cook the pasta according to package directions, al dente (usually 1-2 minutes less).
While the pasta cooks, in a medium sauce pan, heat broth, cream, cream cheese, one cup of the Parmesan cheese, salt, pepper, dry mustard and nutmeg, and cook just to melt and combine. Remove from heat and add to the large bowl.
Once the pasta is cooked, drain and add to the bowl with the other ingredients and stir to combine.
Pour into the prepared casserole dish and cover with the remaining half cup of Parmesan cheese, and bake for 35-40 minutes uncovered or until hot and bubbly and slightly browned on top.
Serve immediately with more chopped parsley as garnish.