Preheat oven (electric cooker: 175°C/circulating air: 150°C/gas: level 2). Wash meat and pat dry. Rub salt and pepper all around the meat. Peel garlic. Use a small, sharp knife to pierce holes in the meat and insert the garlic.
Mix chili flakes with lemon juice and 3 tablespoons of oil. Spread the meat all around with it. Place on the fat pan (deep baking tray) of the oven. Fry on the middle shelf of the hot oven for about 1 1/2 hours.
Gradually add 100-200 ml of water.
In the meantime peel the onions and cut them into thin slices. Peel and wash the potatoes and, depending on their size, cut them in half or into thick slices. Wash thyme, shake dry. Put half of the stalks aside and pluck the leaves from the rest.
Wash the tomatoes and cut into thick slices.
Spread potatoes, onions, tomatoes and thyme stems around the leg of lamb. Drizzle 3 tablespoons of olive oil over it and season with salt and pepper. Fry everything for 1 hour. Turn the vegetables from time to time and after about 30 minutes add the lamb stock.
Grate parmesan. Mix with breadcrumbs, thyme leaves and 1-2 tablespoons of oil. Spread on the leg of lamb and fry for another 15-20 minutes. Cut the meat open. Serve with vegetables and gravy.