Heat your oven to 180°C/Fan 160°C/Gas 4. Slice the frondy bits and roots off the top and bottom of the celeriac. Peel and cut it into chunks. Peel the carrots and chop into thick chunks.
Trim the Brussels sprouts. Pull off any torn outer leaves. Pick the leaves off half the rosemary stalks. Roughly chop the leaves. Peel and cut your onion into wedges.
Tumble the celeriac and carrots into a roasting tin. Spread them out in the tin. Scatter with half the chopped rosemary. Season with salt and pepper. Drizzle with ½ tbsp olive oil. Roast for 15 mins.
Nestle the lamb chops in the tin. Add the sprouts and onion. Roast for another 25-30 mins till the lamb is browned and the veg are tender and charred.
While the lamb and veg are cooking, make your mint sauce. Tip 1 tsp sugar into a small bowl. Pour in 2 tbsp red wine vinegar and 2 tbsp cold water. Stir till the sugar has dissolved.
Finely chop the mint leaves. Add this to the sugar and vinegar mix. Season and stir.
Divide the lamb and roast veg between two warm plates. Sprinkle with the rest of the chopped rosemary. Serve with the mint sauce.