Mix the mince, garlic, lemon zest, chilli, smoked paprika, cinnamon, rosemary, half the parsley, feta and breadcrumbs together in a bowl. Season, then divide the mixture into 8 and roll into balls; rest for 15 minutes.
Heat half of the oil in a large frying pan over a medium-high heat. Add the meatballs and cook all over until browned. Once browned, remove from the frying pan and place in a dish.
In the same pan add a little more oil and sauté the onions until coloured, scraping up the meat that has caught on the bottom of the pan. Add the stock and tomatoes and bring to the boil.
Return the meatballs to the pan and roll them around a bit, then put a lid on and cook over a rapid simmer for 15 minutes. Take the lid off and scatter on the remaining parsley and mint. Keep warm.
For the gratin:
Preheat the oven to 180 C / Gas 4.
Use the butter to grease a 1.5 litre baking dish.
Bring the cream, garlic and thyme to a simmer with a little salt and 120ml water. Lay 1/3 of the sliced potato in the dish. Pour 1/3 of the cream mixture on. Repeat twice more. Lay a sheet of baking parchment on top.
Bake in the centre of the oven for 30 minutes until the potatoes are almost cooked through, then take the parchment off and continue to cook until golden, about 10 to 15 minutes.
Remove from the oven and serve the gratin with the meatballs.