Heat your oven to 180C and roast the potatoes for 1 hour until soft.
While the potatoes are roasting, heat a large frying pan and add half the butter. Add the cabbage and brussels sprouts and cook for about 5 minutes until wilted. Add the spring onions and cavalo nero and stir to wilt. Season well with salt.
Scoop the flesh from the hot potatoes and pass through a potato ricer into a large pot. Add the milk or cream and remaining butter and beat to a smooth mash. Stir through the vegetables and parsley and season well with salt.
Place the colcannon in a serving dish, make a well in the centre and add a little melted butter to serve