In a medium fry pan, melt the butter and oil over medium heat. Add the apples, onion and garlic and sauté till soft, about 15 minutes. Set aside.
Remove from pan and set aside. In the same pan warm the water to a soft boil and add in the celery leaves, coriander seeds and bay leaf. Place the cleaned chicken livers in the warm water and cook on low heat for 10-12 minutes or until they are just cooked through (slightly pink inside).
Remove the livers from the pan with a slotted spoon, and place in a food processor. Place the apple mixture in as well, the butter, calvados, tarragon, salt and pepper. Blend till smooth. It should resemble a thick sauce.
Pour into a terrine dish or your favourite bowl (about 2 cups in volume). With a course pepper grinder, grind pepper over the entire surface, so no pate is peaking through. Cover and place in the refrigerator for at least six hours or overnight.