Heat the butter and oil in a large frying pan. Add the beef and brown on each side, you may need to brown the beef in batches. Place the beef in a large pot and add the stock. Bring to a boil then lower the heat, cover, and simmer for 1 hour. Chop all the vegetables into ½-inch (1 ½ cm) pieces, except the leeks which should be sliced thinly. Add the vegetables to the pot and return to a boil over medium-high heat. Cook uncovered for 20 minutes. Season with salt and pepper. Don’t be tempted to add more liquid as the vegetables will cook down, resulting in a somewhat-thick stew. Serve warm with flatbread or bread, and garnish with chopped parsley.