1. Preheat the broiler and position an oven rack 4 inches from the heat.
2. In a food processor, combine the spinach, herbs, garlic and salt. Add the milk and 2 tablespoons of the oil. Pulse until the mixture forms a pesto like paste.
3. In a bowl, whisk together the eggs. Whisk in spinach mixture, 1/4 cup of the Manchego and the black pepper.
4. Heat remaining oil in a 10-inch ovenproof skillet over medium-low heat. Pour in egg mixture and cook, without stirring, until the frittata sets around the edges, about 5 minutes. Sprinkle with remaining Manchego.
5. Transfer the pan to the oven and broil 1 to 2 minutes until golden and firm to the touch. Slice and serve.