Preheat oven to 375°F.
Wash, dry and chop your vegetables.
Beat the eggs, milk, salt, pepper and about a tablespoon of cilantro in a large measuring cup and set aside. I used 1/4 teaspoon of kosher salt and 1/4 teaspoon of pepper, but you can add more to your taste.
Put an 8” cast iron skillet (or another oven proof skillet) over high heat, add olive oil to coat the pan, about 1/2 a tablespoon (add more if your skillet is not non-stick).
When the oil is hot enough, lower the heat to medium low, add onion and garlic, cook until onion is translucent. Add the chopped tomato and spinach, cook a little bit until the spinach is wilted.
Pour the egg mixture over the vegetable evenly, sprinkle with shredded cheese. I used Mexican blend shredded cheese, but you can use any kind you like or have on hand.
Cook for a minute for the bottom and side of the egg to set. Then transfer the skillet to the oven and bake for 15 minutes until the egg set completely.
Check doneness by shaking the skillet, the middle should jiggle just very slightly. Remove from the oven, garnish with the remaining cilantro and let cool slightly before serving.