Heat butter in 10-inch skillet over medium-high heat. Sauté onions and garlic in butter.
Stir in whipping cream; heat to boiling.
Stir in Parmesan cheese.
Reduce heat. Simmer the sauce uncovered for 30 minutes, stirring frequently.
Heat water and 1 tablespoon salt to boiling in large kettle, add fresh fettuccine. Boil uncovered 2 to 4 minutes, stirring occasionally, until al dente (tender but firm). Begin testing for doneness when fettuccine rise to the surface of water.
Drain fettuccine. Do not rinse. Pour the sauce on the pasta and stir together, dish into servings and top with a little extra Parm and a pinch of nutmeg to make it pretty. Serve with pepper.
I usually serve this with blackened chicken breast and broccoli to add some kind of nutritional value. :) NOTE: I've tried to make this recipe with 1/2 and 1/2 and it's not nearly as good -- sorry calorie counters. It's OK to splurge once in a while.