In a small skillet, melt the butter over medium-high heat. Add the ginger and garlic and cook, stirring, until softened and lightly browned, about 3 minutes. Scrape into a large bowl. Add the lamb, sweet potatoes, curry powder, turmeric, 2 teaspoons salt, and 1 teaspoon pepper and toss well. Cover and refrigerate overnight (or, if you’re doing this day-of, just mix it all in the insert of the slow cooker).