Melt butter in large saucepan.
Saute onions and garlic until lightly wilted (do not brown). Stir heavy cream into butter, simmer for 10 minutes. Stir oysters and juice into mixture.
Simmer for 10 minutes, covered. Stir mushrooms and shrimp, simmer again for 15 minutes covered. Add salt, white pepper and cayenne pepper. Heat 4 quarts of water to a boil. Add fettuccine, stirring occasionally for 12 to 14 minutes. Drain.
Cajun Country -- New Iberia, Louisiana