Clean the French or runner beans, remove the threads if necessary and wash them. Halve the French beans, cut the runner beans into pieces.
Finely chop the onions and sauté in the hot butter until translucent. Add the beans and sauté briefly. Season with salt and pepper. Add the vegetable stock. Cover and cook the beans on a low fire for 15-20 minutes.
In the meantime, reduce the sweet cream and crème fraîche or sour cream in a pan for about 5 minutes by about half. Season with 1 tablespoon of mustard, salt and pepper. Add the leaves of ½ bunch of thyme.
Drain the beans. Arrange on a platter and pour the sauce over them.
Delicious as an accompaniment to fried or grilled lamb chops or grilled sausages.