Wash potatoes thoroughly and cook in boiling salted water for 15 minutes. Meanwhile, cook the beans in a little salted water at medium heat for 10-12 minutes. Peel and finely chop the onion.
Drain the potatoes, rinse with cold water, remove the skin and keep warm. Drain the beans and let them drain well. Drain the fat in a pot, sauté the onion in it until transparent, dust with flour and sauté until golden yellow.
Deglaze with broth and milk and bring to the boil. Season to taste with salt, pepper and sugar. Add potatoes and beans to the bûchamel sauce, fold in and keep warm. Heat oil in a pan and fry the sausages in it until crispy.
Meanwhile wash the parsley, dab dry, put something aside for garnishing and chop the rest finely. Stir the chopped parsley into the béchamel potatoes. Arrange the bûchamel potatoes and sausages on a plate and garnish with the parsley.