Clean and wash the celery and dice very finely. Peel the apple, cut out the core, grate.
Melt 1 tablespoon butter, sauté celery and apple in it, deglaze with stock, add spices and lemon juice, cook on a low heat for 15 minutes. Then puree until creamy. Mix in the creme fraiche and bring to the boil again. Stir in the mustard and honey, remove from the hob.
Crush the butter into flakes with a knife. Then beat this with the whisk under the sauce.