Preheat the oven to 350 degrees F.
Whisk together the eggs, rosemary, red pepper flakes, 1/2 teaspoon salt and 1/4 teaspoon black pepper in a bowl.
Melt the butter in a cast-iron skillet. Add the onions and tomatoes, and saute until the onions are soft and the tomatoes are starting to burst, about 5 minutes. Add the greens, 1/2 teaspoon salt and some pepper; saute for another 2 minutes.
Turn down the heat to low and pour in the eggs. Using a rubber spatula, very gently move the eggs around a bit -- not scrambling them, just massaging them into the cracks and crevasses of the greens.
Once the eggs start to set on the bottom, dollop with the goat cheese and transfer the skillet to the oven. Bake until the frittata is set on top, 5 minutes.