1) Heat a large saute pan over medium heat and add the mustard seeds. Toast seeds until fragrant, about one minute. Add butter and oil. When butter melts, add the turnips and cabbage and saute until cabbage begins to wilt, about five minutes.
2) Mix in vinegar and stock. Bring mixture to a simmer, cover and braise until a knife inserted in a turnip comes out easily and cabbage is wilted and tender, about 25 minutes. Season, to taste, with salt.