In a large saucepan, melt the butter in the olive oil. Add the sliced white onions and stir to coat. Cover and cook over moderate heat until the onions are very soft but not browned, 10 to 12 minutes. Stir in the flour and cook for about 2 minutes. Add the chicken stock and cook, whisking frequently, until the gravy is thickened, 7 to 8 minutes. Stir in the heavy cream and simmer the onion gravy over moderate heat for about 5 minutes.