Rinse fresh spinach and remove the stems. If using frozen spinach, let thaw. Peel onions and finely chop. In a large saucepan, heat butter and sauté onions until translucent. Add spinach to saucepan and mix together. Peel garlic and press through a garlic press. Add garlic to spinach and season with salt and pepper to taste.
In a 20 to 25 cm (approximately 10 inches) pan, heat 2 tablespoons oil and sauté half the pine nuts until golden brown. Meanwhile, press the spinach to remove juices. Whisk half the eggs with half the spinach. Add spinach egg mixture to the pan with the pine nuts and cook over low heat.
Shake the pan to spread the mixture evenly in the pan. Continue to cook over low heat until the eggs are firm. To serve, slide the cooked frittata onto a plate and cut hot or cold in slices. Follow the instructions with the remaining ingredients to create a second frittata .