Wash the beans and cut them into pieces. Cook in a saucepan with plenty of vegetable stock until cooked through.
Pour off some of the cooking water so that there is still enough for a sauce. Thicken with roux (possibly finished product) or starch until the desired consistency. Crumble the feta into small pieces, add the spices and butter and mix well until the cheese is quite melted. Season to taste (possibly with salt and grainy broth) and possibly add a little sugar if it is too sour. It all depends on the feta you use, some are a bit sour.