Wash the fillets, pat dry and cut into 2 cm cubes. Mix salt, pepper, lemon juice and oil in a bowl and marinate the fish cubes in it for 2 hours.
Heat the butter in a large saucepan and sauté the leek and onion cubes in it. Dust the flour over it, pour on the broth and milk and bring to the boil. Add the mushrooms, pepper and fish cubes to the sauce and sauté for 7 minutes. Stir gently so that the sauce doesn`t stick to the bottom. Take the pot off the stove. Mix the egg yolk with the cream and use it to thicken the ragout. Season to taste with pepper and salt.