Preheat oven to 375 degrees F (190 degrees C). Lightly grease a miniature muffin pan.
Melt butter in medium saucepan over medium heat. Stir in the onion and cook until tender and lightly browned.
In a medium bowl, mix together onion, spinach, mayonnaise, mozzarella cheese, nutmeg, salt and pepper.
Unroll buttermilk biscuit dough. Separate each biscuit into two. Place biscuit dough halves into the prepared miniature muffin pan, forming small cups that extend slightly beyond the rim. Fill the biscuit dough cups with desired amounts of the onion and spinach mixture.
Bake in the preheated oven 12 minutes, or until biscuit dough and filling are lightly browned.