Tip the lamb into a bowl. Add 1 tsp each of the sumach and cumin, a pinch of salt and pepper and ½ tbsp olive oil. Turn to coat in the spices.
Warm a pan for 1 min. Tip in the lamb. Fry for 5 mins, turning now and then, till browned all over. Lift out of the pan and pop on a plate. While the lamb cooks, peel and finely slice the onion. Peel and grate or crush the garlic clove.
Once the lamb is on a plate, add the onion to the pan. Season with a little salt and pepper. Cook and stir for 5 mins till the onion has softened and browned. Stir in the garlic.
Add the lamb and any resting juices from the plate back to the pan. Pour in the chopped tomatoes. Add 100ml boiling water. Cover, bring to the boil, then turn down the heat and simmer for 40 mins till the lamb is tender. Taste and add more salt and pepper if you think it needs it.
While the lamb simmers, pour 750 ml boiling water into a pan. Add a pinch of salt and bring to the boil. When it’s boiling, trickle in the polenta, whisking as you add it, till it’s smoothly mixed with the water. Cover and cook over a low heat for 35 mins, stirring now and then, till the polenta pulls away from the sides of the pan.
Tip the walnuts into a dry frying pan. Toast for 2-3 mins till they smell nutty. Tip onto a board and chop.
Add the butter to the polenta and crumble in 75 g of the feta. Stir to mix. Taste and season with salt and pepper if you think it needs it. Spoon the polenta into warm bowls. Ladle the lamb stew over the top. Crumble over 25 g of the feta and top with the toasted walnuts and mint leaves. Serve with a handful of peppery salad leaves.