Argentinian Spinach Empanadas

Ingredients

• 2 tbsp butter
• 2 tbsp all-purpose flour
• 1 1/4 cups whole milk
• 1/2 tsp salt
• 1/4 tsp ground ginger
• 1 tsp garlic powder
• 1/2 medium onion, thinly sliced
• 9 oz (about 4 packed cups) fresh spinach, chopped
• 1 cup grated mozzarella
• 1/2 cup grated Parmesan
• 1/4 tsp nutmeg (optional)
• 16 rounds of flaky empanada dough
• Water, for sealing edges