In a large saute pan, heat pan on medium and add the mustard seeds. Toast seeds until fragrant, about 1 minute. Add butter and oil. When butter is melted, add the turnips and cabbage and saute until cabbage begins to wilt, about 5 minutes. 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.