Peel onion and cut into rings. Wash the fish, dab dry and cut into approx. 1 cm wide pieces. Whisk eggs in a deep plate, season with salt and pepper. Put oat flakes in a deep plate.
Turn the fish first in the egg, then in the oatmeal. Heat the oil in a large pan. Fry the onion rings in it until crispy, drain on kitchen paper. Fry the fish in hot fat for 3-4 minutes on each side.
Boil 500 ml water. Remove the pot from the stove and let it cool down for about 1 minute. Stir in the puree flakes briefly with a wooden spoon. Wash the parsley, dab dry, put some aside for garnishing, chop the rest roughly.
Stir parsley and fat into the puree. Arrange fish fingers with mashed potatoes and onion rings. Garnish with parsley and lemon slices.