1. In large skillet, heat butter over medium heat. When hot, sauté onion until golden, 5 to 8 minutes. Stir in turmeric, ginger, coriander, cayenne, salt, pepper and garam masala, and cook, stirring, for 1 minute. Add yogurt and cook 1 minute more.
2. Stir in turnips, turning to coat evenly with yogurt mixture. Cover skillet, reduce heat to medium-low, and cook turnips until tender, about 30 minutes. Add 1 tablespoon water, if necessary, to keep them moist.