Heat butter and oil in a non-stick pan. When the butter is melted, saute onion for 1 to 2 minutes, then add vegetables. Stir and cook, covered, for 1 minute. Add seasonings, stir, cover, and cook for 1 minute. Add seasonings, stir, cover, and cook for 1 minute. Whisk eggs and milk together, pour into pan, and mix well with the vegetables. Shape to tortilla size. Place one tortilla on the mixture and cover for 1 minute. When the vegetable/egg mixture is mostly cooked through and set, flip the sandwich so the tortilla faces down. Add the cheese, then cover with the other tortilla. After 1 minute, flip and heat the other side. Serve hot. Garnish with fresh cilantro. Makes oe sandwich.