In a medium saucepan melt the butter and cook the onion until softened, about 5 minutes. Stir in the flour and cook for 2 minutes. Stir in the milk and bouquet garni. Bring to a boil, stirring constantly, then reduce heat and simmer for 20 minutes, stirring occasionally. Strain the sauce. Stir in the tomato sauce, nutmeg, salt and pepper.
CREAMY WHITE SAUCE:
In a medium saucepan melt the butter over a moderate heat. Whisk in the flour and cook for 2 minutes, whisking constantly. Pour in the chicken stock and cook, whisking occasionally, until thickened, about 1 minute. In a small bowl whisk together the egg yolks and crÅme fraöche. Stir the cream mixture into the saucepan and heat gently, but do not allow to boil. Season with salt, pepper and fresh lemon juice.
