Simmer vinegar and sugar in a saucepan, then stir in mustard seeds and powder.
Reduce the liquid by half over high heat for three minutes until seeds soften.
Blend the mixture into a creamy purée for several minutes.
Blend in whisky, orange juice, honey, salt, and pepper then set aside.
Preheat the oven to 170°C.
Butter an ovenproof dish and rub with the cut garlic clove.
Layer seasoned potatoes and turnips into the dish and press down evenly.
Whisk cream, milk, salt, pepper, and nutmeg together and pour over the vegetables.
Dot with butter, cover with foil, and bake for one hour until tender.
Increase oven temperature to 200°C and coat the beef in mustard.
Sear the beef in a hot oiled pan then roast in the oven for 90 minutes.
Remove beef from oven, cover with foil, and rest for 15 to 30 minutes.
Uncover the dauphinoise and bake for another 30 minutes until golden.
Reduce meat juices with wine and stock in the roasting pan to create gravy.
Carve the beef and serve with the dauphinoise, mustard, and gravy.