Melt the butter in a large pan. Add the onions and stir through. Add the carrot and celery and about 4 tablespoons of water to help with the cooking.
Cook for about 7 minutes over a medium heat, then add the garlic, cook until fragrant and pour in the wine. Cook off the alcohol, stir in the nutmeg and porcini if using, Dijon mustard and stock powder – add a little more water if the mixture is a little dry.
Pour in the beans with the canned juices and simmer for 10 minutes. Stir in the crème fraiche, taste and season as you wish.