Heat your oven to 200°C/Fan 180°C/Gas 6. Halve the squash and scoop out the seeds. Slice the squash into wedges (you don’t have to peel it but you can if you prefer to). Pop in a roasting tin. Strip the rosemary leaves from their stalks, finely chop them and sprinkle over the squash. Add a pinch of salt and pepper and 1 tbsp oil and toss together well. Roast for 20 mins.
While the squash is roasting, make the mint dressing. Peel the garlic and finely chop or grate it into a bowl. Squeeze in the juice from the lemon. Pick the mint leaves and shred them. Add to the bowl. Pour in 1 tbsp oil and sprinkle in a little salt and pepper. Whisk together with a fork till well combined. Set aside.
Peel the onion and slice it into wedges. When the squash has had 20 mins, remove the roasting tin from the oven. Turn the squash over and tuck the onion wedges in around the squash. Return to the oven for a further 20 mins. Shred the cavolo nero into thin ribbons.
Place a heavy frying pan on a high heat. Sprinkle a little salt and pepper over the lamb steaks and rub them with 1 tbsp oil. When the pan is smoking hot, add the lamb and cook for 2 mins on each side, till golden.
If your frying pan is ovenproof, transfer to the oven to a shelf underneath the vegetables. If your pan isn’t ovenproof, pop the steaks in a small roasting tin. Roast for 8-10 mins, then transfer to a plate to rest, loosely covered with foil, while you finish the rest of the dish.
Put the empty lamb pan back on a medium heat. Slide in the cavolo nero and 3 tbsp water. Cook for 3-4 mins, stirring occasionally, till tender.
Meanwhile, slice the lamb steaks.
Divide the cooked cavolo nero between 2 warm plates, scattered with the rosemary roast veg. Top with a sliced lamb steak each, and spoon over with the minty dressing.