Melt butter in 4-quart heavy saucepan over medium heat. Stir in flour and cook for 1 minute. Add milk, salt and pepper and stir vigorously with wire whip until mixture barely comes to a boil. Reduce heat and simmer 5 minutes, whipping frequently while sauce thickens. Keep warm
Cook bacon thoroughly. Drain on paper towels. Cut into 1/4-inch pieces and stir into sauce
Heat olive oil in large skillet over medium heat. Add sliced mushrooms and minced onions and saute until golden; add to sauce
Cook spaghetti according to the package directions. Drain well and add to the sauce along with the parsley
Blend well and transfer to serving dish. Sprinkle with Parmesan cheese and serve immediately.