Clean the rocket, wash and pat dry. Peel, wash and slice the potatoes. Heat 2-3 tablespoons of oil in a pan. Brown the bacon in it, take it out. Fry the potato slices for approx. 15 minutes, turning them over and turning them golden brown.
Melt the fat in a pot. Add flour and sweat while stirring. Deglaze with stock and milk while stirring, bring to the boil. Stir in mustard. Season to taste with salt and pepper. Peel and chop onions.
5 minutes before the end of the cooking time add onions and bacon to the potatoes, season with salt and pepper. Wash the salmon fillet, dab dry, cut into cubes and sprinkle with lemon juice. Season with salt.
Heat 1 tablespoon of oil in a frying pan. Fry the salmon for about 5 minutes while turning. Arrange the fried potatoes, salmon and rocket on plates. Add a dash of sauce. Serve the rest of the sauce separately.