1) Preheat oven to 375 degrees F. Half cherry tomatoes and toss them with olive oil and a big pinch of salt and pepper. Lay the tomatoes out on a baking sheet, cut-side up and roast them for 25-30 minutes.
2) Whisk the eggs and milk together in a medium bowl.
3) When tomatoes are almost done roasting, add butter to your cast iron skillet over medium heat. Once hot, add sliced onions and a small pinch of salt and cook for a few minutes until onions are soft.
4) Add green chiles and cook for a minute or two in the cast iron skillet.
5) When tomatoes are done, add half of your grated cheese to the skillet and immediately pour in your eggs. Then add your cherry tomatoes evenly distributed and the rest of the cheese on top. Season with a pinch of salt and pepper.
6) Bake the frittata for 14-16 minutes until the center is just set and the eggs are puffed.
7) When done, garnish frittata with slivered basil, slice, and serve!