1. Preheat the oven to 200C fan-forced (220C conventional).
2. For the gravy,add the butter to a frying pan and place over low-medium heat. Once melted and the pan is hot,add the onion and garlic and cook for 5 minutes or until soft and translucent.
3. Add the flour and stir constantly until it comes away from the sides of the pan. It will be gloopy and stick to your spoon,but don't panic. Add the bone broth or stock and using a whisk,whisk any flour clumps until smooth. The gravy will immediately begin to thicken.
4. Add the tomato sauce and apple cider vinegar and whisk to combine. Add the peppercorns and porcini,if using,and turn heat down to low. Simmer for 15 minutes while you cook the chips (**if using regular beef stock,cook for an additional 5-10 minutes to allow the flavour to intensify).
5. Spread the fries in a large non-stick ovenproof fying pan and pop in the oven for 15-18 minutes until golden – some should be quite crisp on the ends.
6. Remove pan from oven and,working quickly,spoon over some of the gravy – you don't want to drown the fries in gravy.
7. Add the chicken pieces and slices of mozzarella to the pan. Return to oven for another 15 minutes or until the cheese is melted and bubbling.
8. Remove from the oven,season with salt and pepper,drizzle over a little extra gravy and serve piping hot with extra gravy on the side for people to help themselves.
*Use the remainder of the chicken in these recipes.
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