Remove any brown parts from the cod. With a mortar and pestle, crush 2 bunches dill and mix with the coarse salt, brown sugar and 1 tablespoon peppercorns. Rub the salt mixture on the cod. Cover with plastic wrap and refrigerate for 1 hour.
Bring the milk to a simmer in a saucepan over medium-low heat. Add the remaining 1 teaspoon peppercorns, the shallots and 1 smashed garlic clove. Rinse the cod to remove the excess salt mixture, then add to the saucepan and cook until flaky, about 10 minutes. Transfer the cod to a bowl and discard the milk mixture.
Preheat the oven to 350 degrees F (180 degrees C).
Cut the garlic head in half horizontally. Drizzle the cut sides with 1 tablespoon oil and season with salt and pepper. Wrap in foil and roast for 30 minutes.
Flake the fish into the bowl with a fork or your fingers. Add the remaining 1/3 cup oil and the roasted garlic, squeezing the pulp from the skins, and mash until combined and creamy. Add the remaining handfuls chives, dill and parsley. Season with pepper. Refrigerate until ready to serve.
For the croutons:
Heat the oil in a skillet over low heat. Add the garlic and let it infuse the oil. Add the bread and toast on both sides. Season with salt and pepper.
To serve, spread the cod mixture on the toast. Serve with lemon wedges.