1. Peel the garlic and crush it with the flat side of a large knife.
2. Peel the potatoes and cut them into pieces. Cook them in salted water with the garlic cloves.
3. Cut the leeks in half and rinse them. Cut them into half-moon slices about 1 cm wide. Melt some butter in a saucepan and sauté the leeks until tender. Season with black pepper, salt, and nutmeg.
4. Gently heat the milk.
5. Drain the potatoes and let them evaporate on the heat. Add the leeks and warm milk. Mash into a coarse puree. Season with black pepper and salt.
6. Melt 40 g of butter in a pan. Whisk in 30 g of flour, then gradually add the fish broth, stirring constantly. Bring to a boil and simmer for 2 minutes to obtain a thickened sauce. Add the cream and season with black pepper and salt. Remove from heat and whisk in an egg yolk and the juice of ½ lemon.
7. Finely chop the chives.
8. Melt some butter in a non-stick pan.
9. Coat the fish fillets in flour and brown them on both sides in the hot butter. Season with black pepper and salt.
10. Serve the leek stoemp with the pan-fried fish. Drizzle with sauce.
11. Garnish with chives. Serve with the remaining lemon.