Peel and cut the sweet potato into roughly ¼-inch cubes.
In a medium mixing bowl, whisk together the eggs, feta, rosemary, salt, and pepper. Set aside.
Preheat the oven to 400 F.
Heat the oil in a large oven-proof skillet on medium heat.
Saute the sweet potato and leek, stirring occasionally, for 10 minutes until the sweet potato is tender but not too soft. Add a small amount more oil if needed to prevent sticking.
Add the greens and saute for 1 minute until wilted.
Pour the egg mixture evenly on top of the vegetables in the skillet. Flatten any elevated pieces with a spatula.
Place the skillet in the oven and bake for about 20 minutes until the eggs are just cooked through and the edges are browned.
Let the frittata cool for 10 minutes to enhance the flavor of the feta, cut it into 8 slices, and serve warm. Alternatively, you can let it cool and serve it at room temperature.