Heat your oven to 200°C/Fan 180°C/Gas 6. Zest the lemon into a small bowl. Squeeze in the juice from 1 lemon half and set aside. In a medium bowl, squeeze in the juice from the remaining lemon half.
Finely chop the rosemary leaves and add them. Peel and grate in 1 garlic clove. Add 1 tsp oregano and
plenty of seasoning. Pour in 1 tbsp oil and mix well.
Scrub the potatoes. Chop them into 2cm cubes. Add to the garlic and herb marinade. Mix together and set to one side.
Trim and chop the aubergine into 1cm cubes. Peel and thickly slice the red onion. Heat a large casserole dish or pan over a high heat. When warm, drizzle in ½ tbsp olive oil. Add the diced lamb and fry for 3-5 mins till golden on most sides. Lift out of the pan with a slotted spoon and pop into a bowl.
While the lamb fries, trim and thickly slice the carrots. Peel and thinly slice 2 garlic cloves.
Keep the pan on the heat and add 1 tbsp oil. Add the aubergine and red onion. Fry for 3-5 mins till the aubergine is browned and slightly softened. Add the carrot and sliced garlic and fry for a further 1 min.
Tip the lamb and any resting juices back in to the pan. Stir together. Drop in the bay leaf and thyme sprigs. Pour in the can of chopped tomatoes. Half-fill the can with water (around 200ml) and add that too. Crumble in the beef stock cube. Stir together. Bring to a bubble and simmer for 40 mins. Stir every now and then. Add a splash of water if you think it's not saucy enough.
While the lamb simmers, tumble the potatoes and their marinade onto a large baking tray. Slide into the oven. Roast for 40 mins till crisp and golden. Turn the potatoes over half way through cooking.
While the lamb and potatoes are cooking, make the green sauce. Take the bowl with the lemon zest and juice in it. Peel and grate the remaining garlic clove into the bowl. Finely chop the flat leaf parsley leaves and stalks and add those. Pour in ½ tbsp olive oil, a good pinch of salt and a crack of pepper and mix together.
Taste and season the lamb stew. Pluck out the bay leaf and thyme sprigs. Serve with the herby lemon roast potatoes and drizzles of zingy parsley sauce.