Heat your oven to 200°C/Fan 180°C/Gas 6. Halve the cabbage. Slice one half into wedges around 21/2 cm thick. Give them a rinse. Pop them in a roasting tin. Rinse the sprouts. Pull off any torn outer leaves. Add to the cabbage.
Drizzle the veg with a little olive oil. Season. Add the sausages. Roast for 30 mins. Shake the pan halfway through to give everything a turn. Fill the kettle and boil it.
Peel the potatoes. Chop them into even-sized, smallish chunks. Peel the apples. Quarter them. Slice off the cores. Roughly chop the apples.
Fill a pan with hot water from the kettle. Add the potatoes and apples. Pop on a lid. Bring to the boil. Turn the heat down. Simmer for 15 mins till the potatoes are soft when you press them with a fork.
Peel and thinly slice the onion. Melt half the butter in a pan. Gently fry the onion over a low heat for 5-8 mins till soft and glossy. Stir in the flour and cook for another minute. Crumble in the stock cube. Turn the heat right down.
Slowly whisk in 250ml hot water from the kettle till you have a smooth gravy. Turn the heat back up. Bring the gravy to the boil. Boil for about 2 mins till it thickens. Taste and adjust the seasoning. Pop on a lid to keep warm.
Drain the potatoes and apples. Tip them back into the pan. Add the rest of the butter and some seasoning. Mash till smooth. Serve with the roast sausages, cabbage and Brussels.