Heat a 10-inch cast iron skillet over medium heat. When hot, add 1 tablespoon vegetable oil. Brown ham. Remove and set aside.
Add remaining 1 tablespoon of vegetable oil to the skillet. Fry onion for 2 minutes. Add potatoes and continue to cook for another 3 minutes. Sprinkle browned ham over potatoes.
Add spring mix to the skillet.
In a medium sized bowl, lightly beat eggs, salt, and ground pepper. Add milk and continue to beat until combine. Pour egg mixture over ham and vegetables in the skillet.
Sprinkle cheese over the top. Place tomatoes cut side up all over the skillet.
Transfer skillet to preheated oven and bake for 30 minutes. Remove and switch oven to broil. Place oven rack to the topmost position. Return skillet to the oven and broil until tomatoes and cheese are slightly brown.