Beat the egg yolks, salt, pepper, mustard, 1 teaspoon lemon juice and sugar in a bowl with the whisk of the hand mixer until foamy. When the mixture is well stirred, first add the oil drop by drop.
When the mixture is creamy, add the oil in a thin stream. Add the rest of the lemon juice and chill. Wash the tomatoes, dab dry, quarter, seed and dice the flesh.
Peel the shallot and cut into fine cubes. Wash the oregano, dab dry and pluck except for something to garnish. Mix tomato cubes, shallots and oregano in a bowl. Season with salt and pepper.
Peel the potatoes and cut them into fine sticks. Heat frying oil in a pot. In the meantime season medallions with salt and lemon pepper. Heat 1 tablespoon of oil in a pan and fry the medallions on both sides for about 5 minutes.
Put potato pegs into the hot fat and deep-fry. Let it drip off on kitchen paper. Place in a bowl and salt. Arrange steaks on plates. Add tomatoes and serve garnished with oregano.
Add french fries and lemon mayonnaise. Garnish with a slice of lemon if desired.