Heat your oven to 180°C/Fan 160°C/Gas 4. Pull a clove of garlic from the bulb. Peel and grate it. Place the rest of the bulb in a dish and roast in the oven for 30 mins.
Cut the lamb into 2 inch cubes. Heat 1 tbsp oil in an ovenproof pan or casserole dish and add the lamb. Cook for 4-5 mins till browned on all sides. Transfer to a bowl using a slotted spoon.
Cut the aubergine into similar sized cubes as the lamb and add to the pan with the lamby oil. Cook till coloured on all sides, turning regularly. Lift the aubergine out and put in the same bowl as the lamb.
While the garlic roasts, slice the fennel into thick slices. Heat 1 tbsp oil in the pan and add the fennel. Cook for 2 mins on each side till slightly caramelised. Tip the lamb and aubergines back into the pan. Add the grated garlic. Zest the orange straight into the pan. Set the orange aside for later. Add the olives. Season.
Stir to combine then tip in the chopped tomatoes. Half fill the can with water and add to the pan/casserole dish. Pull the leaves from the rosemary and stir through. Bring to a bubble. Cover. Slide into the oven for 45 mins.
When the lamb has cooked for 20 mins, cook the polenta. Put 750ml boiling water in a large pan over a medium heat. Swirl the water and slowly pour in the polenta. Whisk till thickened. Turn down the heat. Stir every 3-4 mins. After 15 mins the polenta will be thick and creamy. Cover to keep warm.
Squeeze the roasted garlic cloves from their skins. Crush to a paste with the back of a knife. Cube the butter. Vigorously beat the garlic and butter into the polenta and generously season.
Roughly chop the parsley leaves. Squeeze a little orange juice into the lamb stew to taste. Serve the orangey lamb on a bed of polenta. Scatter over the parsley.