In a small heat-resistant, non-metallic bowl melt butter in Microwave Oven 30 seconds.
Add onions and heat, uncovered, in Microwave Oven 4 minutes or until onion is lightly browned. Stir occasionally.
Cook spinach. Drain well.
Combine browned onion, spinach, salt, pepper, nutmeg and cream cheese in a shallow, 1 1/2-quart, heat-resistant, non-metallic casserole. If desired, add milk to make a thinner mixture.
Heat, uncovered, in Microwave Oven 5 to 6 minutes or until heated through.