In a large nonstick skillet, sauté the 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 for 3-4 minutes or until bread is browned and cheese is melted, turning once. Cut each sandwich into quarters.