Salt the cod very generously with kosher salt. Place in a perforated pan or colander and let sit, refrigerated, for 2 days.
Add the herbs, spices onion and garlic cloves to the milk and heat for 5 minutes, do not let boil. Add the cod and poach until tender, around 5 to 7 minutes.
Gently remove the cod with a slotted spoon. Remove any bones and skin. Meanwhile peel and chunk the potatoes. Cook in well-seasoned water or better still, use the milk the fish was poached in. When the potatoes are tender drain, and puree them, either in a ricer or a mixer using the paddle attachment. Pull out 1/3 of the potatoes and add the cod.
Mix well, at medium speed if using a mixer, drizzling in the olive oil and a bit of the cooking liquid. Keep mixing and adding liquid until the desired consistency is reached. Pound the green garlic or 2 cloves of garlic to a fine paste in a mortar and pestle and stir into the puree.
For an even more full flavored brandade, pass the onion, garlic, and aromatics through a food mill and stir some of this puree in as well. Taste and adjust the seasoning as needed
Recipe courtesy of Kelsie Kerr/Alice Waters, Chez Panisse - Berkeley