This popular dish can be served as an entree or a main.
Use two 800ml casserole dishes or one 1500ml casserole dish.
Preheat oven to 180C.
In a small saucepan, gently simmer the milk, cream, rosemary, garlic, bay leaves and onion with cloves for 15 minutes.
Grease the bottom of the casserole dishes with butter.
Slice the potatoes thinly with their skins on and arrange a single layer of potatoes over the bottom of the dish. Alternate with a layer of onions. Continue with each layer in both of the casserole dishes until you have used up all of the potato and onion, or until you have 1 cm left at the top of the dish.
Strain the milk mixture and season. Pour over the potatoes and cover with foil. Bake for 1 hour, covered.
When the potatoes have cooked through, remove the gratin from the oven and take off the foil.
Lay the coppa slices on top of the gratin. Sprinkle the grated parmesan and oregano leaves over the top and place back into the oven, uncovered for 10 minutes or until the coppa is crisp.
Remove from the oven and allow to sit for 10 minutes before serving.