Melt butter in a small pan over medium heat. Add onion, garlic, thyme and pinch of salt and pepper and sauté for five minutes or until onion softens.
In a large mixing bowl, combine pork mince, chicken and thick cut bacon. Add salt and pepper, all spice, Dijon mustard, sage, parsley, egg, sherry and the cooked onion and garlic mixture. Mix well.
Line terrine mould with cling film. Lay bacon rashers all the way along the sides. Fill with mould with stuffing, pushing it down as you go to remove any air bubbles. Take the ends of the bacon and place them across the filling. Push the bay leaves into the top of the terrine and cover with lid. Press the terrine over night to force out any remaining air bubbles.
Preheat oven to 150C and cook the terrine in a bain marie for 1 to 1 and half hours or until 72C inside.
To make the pickled onions, combine the onion, vinegar and salt in a large mixing bowl. Cover with plastic wrap and leave to soak for at least 15 minutes. Transfer the onions to a clean bowl, add parsley, drizzle with olive oil and serve.