Bring pot of salted water to boil. Add pasta, boil 2 minutes, stir, cover and turn off burner. Pasta will continue cooking while you make the mushroom sauce
Cook pancetta in skillet over medium heat, stirring frequently until crispy. Remove from pan and drain on paper towels. Brown mushrooms in fat rendered from pancetta or in butter an olive oil over medium high heat until juices release and they turn golden, 5-6 minutes. Add white part of the onions and cook until softened. Stir in flour and cook one minute, then add broth and deglaze pan. Reduce heat to medium low and cook for 10 minutes. Add sour cream and Fontina and season with pepper. Simmer gently until cheese melts, stir and add pancetta back to pan.
Drain pasta and add to sauce**. Stir to combine. Serve immediately, garnished with green onion tops and chopped parsley. OR turn into a 1 1/2 quart buttered casserole dish, sprinkle with parmesan and refrigerate until ready to bake. Bake uncovered at 325 for 40-45 minutes or until heated through. Garnish with green onions and parsley. Makes 4 servings