Heat the oil in a large pan, add the onion and cook for 10 mins until soft. Add the garlic and cook for 1 min more, then tip in the lamb mince, turn up the heat and brown, breaking up the mince with a wooden spoon. Once the lamb is browned, add the cinnamon, crumbled stock cube, tomatoes and oregano. Stir well and simmer for 15 mins, covered, then for 15 mins, uncovered, until the sauce has thickened and the tomatoes have cooked down.
Meanwhile, cook the penne for 25 minutes until al dente, drain well. Heat oven to 200C/180C fan/gas 6. Mix together the ricotta, most of the Parmesan and the milk with plenty of seasoning, and stir into the penne. Tip the mince into a large baking dish, top with the macaroni mixture, and scatter over the remaining Parmesan. Bake for 30 mins until golden and bubbling. Serve hot with a fresh side salad.