An 1/8th tsp each ground cinnamon, clove and nutmeg
For the Guinness Mustard, combine all ingredients in a non-reactive bowl, cover with cling film and allow to stand for 2 days at room temperature. Process in a food processor to the desired consistency.
Preheat the oven to 160 degrees Celcius. Blanch the spinach, for a few seconds in boiling water, refresh in iced water. Drain the spinach well and squeeze out any excess liquid. Finely chop and set aside. Boil the whole eggs for 9 minutes and then plunge into iced water. Peel and set aside.
Combine the spinach with all the remaining ingredients for the terrine except the whole boiled eggs. Grease the inside of a terrine dish with butter and press in half of the chicken mixture, making sure to cover the sides. Trim the tops and bases from the eggs and arrange them along the length of the terrine so that they sit flush against each other. Cover the eggs with the remaining mixture, ensuring that it gets into all the gaps. Cover with foil or a lid.
Place the terrine dish into a roasting tray and fill the tray with hot water halfway up the side of the terrine dish. Bake for 1 hour and test with a skewer. If the juices run clear, the terrine is cooked. If the juices are slightly pink, continue to cook for a further 15 minutes or until cooked through. Remove from the oven and allow to cool at room temperature.
Refrigerate the terrine and press down with a heavy weight. (You can use a piece of cardboard cut to the size of your dish and covered with foil. Place unopened cans on top of the cardboard to provide weight.)
Slice the terrine and serve with the Guinness Mustard, cornichons and crusty bread.