Preheat the oven to 350°F. In a large bowl, whisk together eggs, milk, salt, and pepper. In a skillet, sauté chopped asparagus and onions in butter until tender. Pour the egg mixture over the vegetables and cook for 2-3 minutes. Transfer the skillet to the oven and bake for 10-15 minutes, until the frittata is set. Serve hot or cold.
