In a large nonstick skillet, saute onion in oil until tender. Add garlic; cook 1 minute longer. Stir in the spinach; cook and stir until wilted. Drain. Stir in the feta, pine nuts, salt, pepper and nutmeg.
Spread over 4 bread slices; top with remaining bread. Spread outsides of sandwiches with butter. Grill, uncovered, over medium heat until bread is browned and cheese is melted, turning once, 3-4 minutes. Cut each sandwich into quarters.