Heat your oven to 200°C/Fan 180°C/Gas 6. Fill a pan with the boiling water and a pinch of salt. Heat to bring back to the boil. Scrub the potatoes and chop them into chunks around 1-2cm across. When the pan is boiling, add the potatoes and simmer for 5 mins.
While the potatoes simmer, halve the pepper and slice it into 2cm chunks. Pop in a large bowl. Peel the red onion and slice it into around 8 thick wedges. Add to the bowl too.
Peel and finely chop the garlic cloves. Pick the leaves off the thyme sprigs (if you have a metal sieve you can push the sprigs through the sieve, which will quickly strip the leaves off their stalks. Add them both to the bowl with 1 tbsp olive oil and a pinch of salt and pepper. Turn everything to coat it in the seasoned oil.
Spread the veg out in a large roasting tin. Drain the potatoes and toss them into the tin. Turn to mix everything together. Nestle the sausage wheels in the tin. Slide into the oven and roast for 30 mins, till the sausages and veg are browned and tender.
While the sausages and veg roast, crumble half the stock cube into a heatproof jug. Add 1 tbsp mustard. Pour in 200ml boiling water and stir to mix well. Trim the dry ends off the cavolo nero and finely slice the leaves.
Take the veg and sausages out of the oven. Turn the oven down to 180°C/Fan 160°C/Gas 4. Pour in the veg stock. Tuck the cavolo nero into the tin. Return to the oven and roast for another 10 mins, till the cavolo nero is wilted.
Serve the Cumberland sausage wheels with the roast veg and any juices from the roasting tin spooned over the top.