In a large skillet or sauté pan, heat 1 tablespoon of the oil over medium heat. Add the onion, garlic, ginger, and a pinch of salt and cook, stirring until golden, 5 to 7 minutes. Scrape into a bowl and set aside until evening (refrigerate and reheat, if you’d like).
In the same skillet, heat the remaining 1 tablespoon oil over medium-high heat. When it shimmers, add the lamb and sprinkle it with the coriander, turmeric, cayenne, and 1 teaspoon salt. Cook, turning occasionally with a thin metal spatula, until the lamb is browned and crusty on one or two sides, 3 to 5 minutes. Transfer to the slow cooker and pour ½ cup (120 ml) water into the pan and stir to scrape up any browned bits; pour the liquid over the lamb. Pile the turnips on top. Cover and cook on low for 8 hours.
Using a slotted spoon, transfer the lamb and turnips to a bowl. With an immersion blender, blend in the yogurt a little at a time, then the garam masala. Season with more salt, if needed. Return the lamb and turnips to the sauce and serve topped with the onion mixture.