Prepare 20 pieces of charcoal in a charcoal chimney (or reserve coals from the campfire) and let burn until they are white around the edges.
Diced onions, garlic, green onions and broccoli (or other veggies of your choice). Set aside.
Whisk together eggs and whole milk. Add salt and pepper. Set aside.
Place 5-8 coals underneath your cast iron skillet and heat oil (or butter) in the skillet.
Add the onions and broccoli to the skillet and sauté until tender. (Cover with the lid to speed up the cooking time of the broccoli.) Add garlic and green onions and sauté for 1-2 minutes.
Spray skillet with cooking spray (or use a paper towel to add oil to the sides of the skillet). Pour egg mixture into the skillet and fold in cheddar cheese.
Cover with cast iron lid and arrange 12-15 coals on the top of the skillet.
Allow to cook for approximately 10-15 minutes. It is fully cooked when there is no more liquid egg mixture in the center of the frittata.