1/4cupcrumbled feta cheeseplus additional for serving
Place a rack in the center of your oven and preheat the oven broiler to high.
Place the chopped sweet potato in a microwave-safe bowl. Fill the bowl with water to a depth of 1/2 inch. Cover the plate with a paper towel (or use a reusable microwave-safe cover). Microwave on high until the cubes are slightly tender but not fully cooked, about 4 to 5 minutes. Let cool a few minutes, then drain.
Meanwhile, heat 2 tablespoons oil in a 10-inch cast-iron or similar ovenproof skillet over medium-high heat. Swirl to coat the skillet. Once the oil is hot, add the ham. Cook, stirring often, until the ham is browned on all sides, 4 to 5 minutes. With a slotted spoon, remove the ham to a plate and set aside.
Reduce the heat to medium. Add the remaining 1/2 tablespoon oil, onion, rosemary, and sweet potato. Sauté, stirring often, until both the onion and sweet potato are lightly browned and the onion is softened, about 7 minutes.
In a large mixing bowl, whisk the eggs until blended. Whisk in the cheddar, black pepper, and salt.
Remove the skillet from the heat. Slowly and gently pour in the egg mixture. Sprinkle with the ham and feta.
Place the skillet in the oven. Broil until the eggs are set and the top is golden, about 3 to 5 minutes. Watch very carefully towards the end of the cooking time to ensure it doesn’t burn (don’t walk away!). This time may vary depending upon your skillet (different materials hold heat differently). If at the 5 minute mark your frittata is not fully cooked, switch the oven to BAKE at 350 degrees F and continue cooking until the eggs are set. Remove from the oven (be careful, the skillet handle will be hot). Let cool a few minutes. Slice and serve warm or at room temperature.