1. Press the cold boiled halibut through a fine-mesh sieve into a bowl, working it to a smooth paste. Set aside.
2. Scald the milk (a technique in which milk is heated just below boiling, 180-185°F / 82-85°C, until small bubbles form around the edge of the pan and steam rises steadily) with the onion slice and blade of mace in a medium saucepan (2-quart / 1.9 L) over medium heat, 5-8 minutes. Do not allow the milk to boil.
3. Remove the onion and mace from the milk with a slotted spoon and discard them. Stir the sieved fish into the hot milk and reduce heat to low.
4. In a small skillet or saucepan, melt 1 1/2 tablespoons (22 g) of the butter over medium-low heat. Whisk in the flour and cook together, stirring constantly, for 1-2 minutes until the mixture turns a pale straw colour and smells faintly nutty — this is a beurre manié-style roux. This step binds the soup.
5. Gradually whisk the butter-flour roux into the hot milk and fish mixture, stirring constantly. Continue to cook over low heat for 4-6 minutes, until the soup thickens enough to lightly coat the back of a spoon and no raw-flour taste remains.
6. Add the salt, pepper, and the remaining 1 1/2 tablespoons (22 g) butter in small pieces, stirring gently after each addition until fully incorporated and the soup is glossy and uniform. Taste and adjust seasoning.
7. Ladle into warmed bowls and serve immediately.
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