Wash potatoes thoroughly and cook in boiling water for about 20 minutes. Drain, rinse with cold water and peel. Peel onions and cut into rings. Dab meat dry, cut the fat edge several times.
Season slices of meat with salt and pepper. Cut potatoes into slices. Heat 3 tablespoons of oil in a frying pan and fry the potato slices for about 10 minutes, turning them over. Season with salt and pepper.
In the meantime, heat 2 tablespoons of oil in a 2nd frying pan. Brown the meat on both sides. Add butter and fry for another 2-3 minutes at medium heat. Remove meat and keep warm.
Dust the onions with flour and fry them in frying fat until light brown. Deglaze with stock, bring to the boil and simmer for 1 minute. Season to taste with salt and pepper. Serve the roast beef with the onion sauce. Wash parsley, shake dry and cut into strips except for a little bit for sprinkling.
Sprinkle roast with parsley and garnish. Green salad tastes good with it.