1. Preheat oven to 450 degrees. Spray mini-muffin tin with cooking spray. Heat oil in a saute pan and cook onions over medium heat, stirring frequently, until they begin to caramelize.
2. Add asparagus, turn up heat, and cook until bright green and just tender - about 2 - 3 minutes. Remove pan from heat.
3. In a medium bowl, add eggs and milk and beat until well combined. Fold in cheese and add vegetables to egg mixture. Optional: add salt and pepper to taste.
4. Spoon egg mixture into muffin tin (filling each cup 1/2 - 2/3 full); place in oven and cook 8 - 10 minutes, or until golden brown.