chicken: wings removed and reserved for another use
Put garlic and milk into a small pot and bring just to a boil over high heat. Immediately reduce heat to medium-low and simmer until garlic is very soft, 20–30 minutes. Transfer to a large bowl and set aside to cool; then mash with a fork to a smooth paste and set aside.
Meanwhile, lay 1 of the chickens breast side down. Using a sharp knife, slit skin the length of the backbone, then pull skin off chicken in 1 piece and set aside. Repeat process with the remaining chicken. Bone chickens and cut meat into 1.25cm dice
Preheat oven to 180°C. Add diced chicken, nuts, basil, thyme, lemon zest, salt, and pepper to garlic paste in bowl and mix well. Line bottom and sides of a 6-cup terrine with chicken skin, draping it over sides. Pack chicken mixture into terrine, pressing it flat with your fingers. Fold skin over chicken mixture to wrap completely.
Cover terrine with a double layer of aluminum foil. Set terrine in a deep baking pan and add enough boiling water to the pan to reach two-thirds of the way up the sides of terrine. Bake until internal temperature of terrine reaches 70°C, about 1 hour.
Remove pan from oven, take terraine out of pan, pour off water, and return terraine to pan. Remove foil and let terraine cool briefly, then cover with plastic wrap. Place a piece of heavy cardboard, cut to fit, inside terrine. Weight terrine with 2 or 3 heavy cans (some juices may spill over) and refrigerate overnight. Umnmold terrine onto a serving platter, garnish with a sprig of fresh thyme, if you like, and spoon some of the gelée around the terrine before slicing.