2 rounded tbsp hot horseradish
1 small onion, peeled and cut in half
1 level dessertspoon plain flour
25g/1oz plain flour (about 1 heaped tbsp)
2 level tsp wholegrain mustard
salt and freshly milled black pepper
1 level dessertspoon mustard powder
approx 1 litre/1 and three quarter pints vegetable stock, or water from the potatoes
1 heaped tbsp cr猫me fra卯che
3-rib joint, wing end or sirloin of beef on the bone (approx 2.7kg/6lb)