Put the cleaned and washed pork trotters in a pot with water, heat to boiling point and discard this first water. Wash them again, pour in new water, add the onion head, sliced into 4 but not all the way through, and the carrots cut in 2, add the black pepper grains.
Boil until the meat comes off the bones. Strain the soup, remove the onions, carrot and black pepper grains (they've done their job), cut the deboned meat finely and return to the soup.
Put on low heat to come to a boil. Separately, beat the yoghurt with the flour and a little cold water to a smooth sludge. In lots, pour spoonfuls of the soup to it, in order to even out the temperatures, next pour this thickening agent into the main soup.
Leave to come to a boil again and remove the pot from the heat.