Sauce: Peel avocado and remove stone. Dice flesh and sprinkle with lemon juice. Add avocado cubes into cold beef stock and mash them. Stir in sour cream and season to taste with salt, pepper and lemon juice.
Divide salmon into 4 portions, spice with salt, pepper and lemon juice. Heat oil in a pan and fry salmon on each side. Add 1 tsp. butter some minutes before finished. Serve with sauce.