Clean leeks, slit lengthwise and rinse thoroughly. Cut into 1/2cm wide rings. Blanch in boiling salted water for 1 min, rinse with ice and drain well.
Remove skin and bones from halibut and cut into bite-sized pieces. Sprinkle all over with juice of one lemon, season lightly with salt, season with pepper and sprinkle with oregano.
Grease a fireproof dish. Alternate layers of leek and halibut. The last layer should be halibut.
Mix well egg yolks with crème fraîche and parmesan, season with salt and season with pepper. Spread evenly over the fish.
Bake at 200 °C for about 25 minutes. Serve with salad and mashed potatoes.
G&U