Heat the oven to 180°C. Dust the duck legs with flour and season with salt and pepper. Heat a heavy-based ovenproof sauté pan (or flameproof casserole) over a medium-high heat and add a good drizzle of olive oil. When hot, add the duck legs to the pan and colour for 4–5 minutes on each side. Remove and set aside. Add the carrots, celeriac and onion to the pan, lower the heat and sweat for 4–5 minutes. Now add the bouquet garni and the orange zest and juice. Let bubble to reduce the juice down until the pan is almost dry. Pour in the red wine, bring to the boil and skim off any scum from the surface. Season with salt and pepper. Return the duck legs to the pan, immersing them in the wine. Put a lid on the pan and cook in the oven for 1 hour or until the duck legs are tender but still holding their shape. In the meantime, prepare the polenta. Bring the milk to the boil in a heavy-based saucepan. Season with salt, pepper and nutmeg and add the butter. Add the polenta in a steady stream, stirring as you do so, and cook, stirring frequently, for 15–20 minutes. When the duck legs are cooked, remove them from the pan and set aside on a plate. Over a medium-high heat, let the braising liquor bubble rapidly to reduce by about half, then add the butter, chopped olives and basil. Discard the bouquet garni and orange zest. Return the duck legs to the pan and spoon over the reduced liquor. Serve with the polenta.