Preheat oven to 350 degrees. Grease 4 sections of a muffin pan with the butter.
Mix the eggs in a bowl, adding the milk to the eggs. Divide the onion and kale pieces evenly among the 4 muffin sections that are greased.
Pour the egg-milk mixture into each of the vegetable-filled muffin sections, filling about 3/4 of the way. Top with shredded cheese, and season with salt and pepper to your taste.
Bake for 20 minutes. Allow to cool, remove from muffin pan, and eat warm or store in the refrigerator for up to 3 days.
Other fun combinations: 1. feta, spinach, and onion 2. artichoke, asparagus, and mozzarella 3. black beans, salsa, and jack cheese 4. mushrooms, spinach, and Swiss cheese