Melt butter in a saucepan over medium heat. Add onion and cook until tender; drain. In a bowl, combine onion, spinach, mayonnaise, cheese and nutmeg; set aside. Unroll biscuit dough; divide each in half horizontally. Press biscuit halves into lightly greased mini muffin cups. Shape each into small cups that extend slightly beyond the rim. Fill with spinach mixture. Bake at 375 degrees for 12 minutes, or until biscuits are lightly golden. Serve warm or at room temperature. Makes 16.