Cut the meat into pieces of about 1½ cm. Heat 2 tablespoons of olive oil in the casserole and fry the meat on a high heat for 5 minutes until golden brown. Sprinkle with salt and pepper to taste and remove the meat from the pan with a slotted spoon.
Heat the rest of the oil in the pan, add the leek and stir-fry for 2 minutes. Put the meat back into the pan together with the vinegar, 500 ml chicken stock, half the parsley and the bay leaf. Bring to boil. Put the lid on the pan and simmer the meat for about 2 hours until fully cooked (longer if necessary). Use the smallest burner on the stove and if necessary, insert a flame distributor. Stir occasionally. Add some extra hot chicken stock if necessary.
Boil the beet for about 25 minutes. Drain and let them cool slightly. Peel off the skin.
Season the stew with salt and freshly ground black pepper. Cut the remaining parsley. Add the beets 2 minutes before serving. Crumble the feta over the stew, and garnish with walnuts and parsley.
Beets are available all year round. In the summer they are a bit smaller and taste fresher. If you use winter beets, you should cook them longer, about 40 minutes.