How to Make It
In a saucepan, simmer the milk with the garlic, salt, potato, bay leaves and thyme, covered, until the potato is tender, 15 minutes. Remove from the heat and add the whitefish; let stand for 5 minutes. Fine-strain the mixture, reserving the milk; discard the herbs.
Preheat the broiler. In a food processor, blend the fish, potato and garlic, adding the reserved milk 1 tablespoon at a time, until a thick puree forms. Blend in the 1/4 cup of olive oil in a thin stream. Add 3/4 cup of the grated cheese and pulse just until incorporated.
Grease a 2 1/2- to 3-cup baking dish. Spread the whitefish brandade in the dish and drizzle with oil. Top with the remaining cheese. Broil the brandade 6 inches from the heat until golden, 10 minutes. Serve with grilled bread.