Preheat oven to 425 degrees. Truss chicken and coat all over with a thick layer of mustard. Bake in a small gratin dish until mustard turns golden brown, about 40 minutes.
In a small pan heat creme fraiche. Take chicken out of oven, remove any surplus mustard, and pour the creme fraiche over. Bake 10 minutes longer. Remove chicken to a hot dinner plate. Whisk the sauce, season to taste with salt and pepper and whisk in butter. Pour over chicken and serve