Clean the mussels well under running water to get away the sand and dirt.
Cut the onion and garlic in small pieces.
Cut the celery and carrot in small cubes.
Cut the leek in thin slices.
Melt the butter in casserole and cook the vegetables, onion and garlic for 5 minutes.
Sprinkle the flour over the mixture and stir well.
Add the mussels and white wine and cook on high heat for 10 minutes or until the mussels are open. Add a sprig of thyme, salt and pepper. Cover the casserole while cooking them to get the steam.