Line a baking sheet with parchment paper. Arrange potato slices in a thin, slightly overlapping layer and scatter shallot rings over the top in an even layer. Drizzle with 2 tablespoons of the olive oil and season lightly with salt and pepper. Bake 20 minutes or until vegetables are cooked through. Remove from oven and set aside.
Reduce oven temperature to 350˚F.
In a medium bowl, whisk together eggs, milk, salt, pepper, rosemary and parmesan cheese until well combined; stir in chopped prosciutto; set aside.
Heat remaining 1 tablespoon olive oil in a 10" cast iron skillet over medium heat and swirl to coat. Layer potato and shallot mixture over the bottom; pour egg mixture over all. Transfer to oven and bake 30-35 minutes, or until the frittata has puffed and the center is set.
Let cool 10 minutes before slicing in to wedges. Serve.