Wash, quarter and seed the tomatoes. Cut the flesh into small cubes. Wash fish, dab dry and cut into pieces of about 1 cm width. Whisk eggs in a deep plate, season with salt and pepper.
Put the oat flakes in a deep plate. Turn the fish first in the egg, then in the oatmeal. Heat oil in a large pan. Fry the fish for 3-4 minutes on each side. Bring 500 ml water to the boil.
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. Melt the fat in a pan, toss the tomatoes in it for 1 minute.
Mix in half of the parsley and season with salt. Stir the rest of the parsley into the purée. Arrange the fish fingers with mashed potatoes and tomato butter. Garnish with parsley and lemon slices.