First make mashed potatoes. Boil water in a large pot. Peel and cut potatoes in quarters and place in the pot. Cook until soft. Drain the water. Add milk and butter 1 tablespoon of butter. Mash. Add more milk if needed.
Heat 1/2 tablespoon of butter in the pan and olive oil. Add 1/2 cup of beef broth. Add sausages and brown them on both side, cook until done, about 7 minutes.
Take sausages out and add more butter into the pan. Add sliced onions. Cook until fragrant, about 5 minutes. Add flour and mix very well. Add leftover broth, stir and let cook over low heat for a little, until thickened. Add sausages and cook for another 2 minutes.
use more broth if you feel that the gravy is too thick