In a medium saucepan, melt the 2 1/2 tablespoons of butter. Add the chopped onion and the remaining 1 teaspoon of thyme and cook over moderately low heat, stirring, until softened, 5 minutes. Add the flour and cook over moderate heat, whisking constantly, until a light golden paste forms, 3 minutes. Whisk in 1 cup of the milk and cook, whisking, until very thick and no floury taste remains, 8 to 10 minutes. Remove from the heat and whisk in the cream, crème fraîche, sugar, 2 teaspoons of salt, 1/2 teaspoon of pepper and the remaining 1 1/2 cups of milk. Let the béchamel cool.