The potatoes are peeled, washed and cut into large blocks. Chop the onion finely and fry it in a pot with a little oil.
In small bowl, put 2 tablespoons flour and dissolve with water - it shouldn't be too thick nor too runny, it has to be like a sludge.
Add the resulting sludge, while stirring continuously, into the pot, leave it to boil a little.
Remove from the heat and serve.