Heat 1 Tbsp avocado oil in a large pan on medium-low heat, and add sliced onion when hot.
Cook onions on medium-low heat for approximately 30 minutes, until soft and browned, stirring every 5 minutes.
Meanwhile, preheat oven to 350 degrees Fahrenheit and heat a second pan on medium heat.
Add remaining 1 Tbsp avocado oil to the second pan, and add the diced bell pepper, mushrooms, and spinach to pan when hot.
Saute vegetables for approximately 7-10 minutes, until soft.
Spray a silicone muffin tin with avocado oil cooking spray.
Add small spoonfuls of cooked veggies, caramelized onions, and a sprinkle of shredded white cheddar cheese to each tin.
Whisk all eggs in a large bowl or in a large measuring cup with a spout, and pour in equal amounts over each muffin tin.
Bake at 350 for approximately 25-30 minutes. Check for doneness by inserting a knife into an egg muffin. Ensure eggs are completely set.
