In a 14- to 15-inch wok or 6- to 8-quart pan over high heat, stir butter, currants, lemon, and sugar until lemon is tinged with brown, about 3 minutes. Scrape into a bowl.
Add spinach leaves and nutmeg to wok. Stir often over high heat until spinach is wilted and browned bits in pan are released, 2 to 3 minutes. Stir in currant mixture and salt to taste. Pour into a bowl.