This is a Dutch recipe for a sour-ish sauce (quite similar to a béchamel), which is traditionally served with brown beans, boiled potatoes, fried onions and fried salt pork, and romaine lettuce, sometimes all mashed up together, like the Dutch like so much...
The recipe
Depending on how much sauce you'd like to drown you beans in this could serve maybe 2 to 4 people.
1 tablespoon of butter
1 tablespoon of flour
up to 1 cup buttermilk
Melt the butter in the pot, stir in the flour and briefly cook it, but do not let it burn. Then gradually whisk in the buttermilk. Take care that it does not come to a boil, or otherwise the sauce will separate.
Add salt and pepper to taste, and chives for some color.