Peel and dice potatoes. Place in a saucepan and cover with cold water. Bring to a boil, add salt and simmer for 20 minutes. Meanwhile, prick sausages with a fork, heat a skillet and brown the sausage in 1 tablespoon oil. Slice the onion. Remove the sausage to a plate and add the onion to the pan. Cook onions until soft, then add 2 tablespoons flour, cook for 1 minute. Add the chicken stock and return the sausage to the pan, cooking it through for about 15 to 20 minutes.
Drain the potatoes and run them through a food mill into a saucepan. Add the cream, butter, creme fraiche and mustard. Season with salt and pepper. Serve with side of grainy mustard for dipping.