Lamb steaks with chats & Brussels sprouts

For ➍
600 g chat potatoes*, halved
350 g Brussels sprouts
40 g butter, chopped
8 trimmed lamb steaks
1 small onion, chopped
6 cl white wine
12.5 cl beef stock
1 ts Dijon mustard
1 tbs chives, chopped

Boil potatoes and sprouts, separately, in medium pans of boiling water, uncovered, until tender. Drain.
Meanwhile, melt half the butter in a large frying pan, add the steaks and cook for about 3 minutes on each side or until cooked as desired. Remove from the pan, cover to keep warm.
Add onion to the same pan, cook, stirring, until softened. Add wine to the pan, then add the stock and mustard and simmer, uncovered, until hot.
Toss remaining butter and chives through the hot potatoes.

Serve lamb with the sauce, potatoes and sprouts.

*Or small new potatoes.