Preheat oven to 350°F. Cube 1 yellow potato (skin on), aiming for about 1 cm x 1 cm. Trim and discard the green tops and root ends of 1 leek, then thinly slice. Trim and discard the woody ends of 200g asparagus, then roughly chop, aiming for 1-inch pieces.
Heat a well-seasoned cast iron on medium-low, once hot, add butter 2 tbsp unsalted butter and 1 tbsp olive oil. When the butter begins to foam, add the cut potato, leeks and asparagus. Season with ½ tsp salt. Cover, leaving room for steam to escape, and let cook on medium-low, stirring occasionally to prevent burning, about 10 minutes.
Meanwhile, crack 6 eggs into a medium bowl. Cover with 1/4 cup whipping cream and season with ½ tsp salt and lots of cracked black pepper. Whisk well — there should be no streaks and the mixture should extremely smooth. Grate in 150g low-moisture mozzarella and stir until well-incorporated.
Once the veggies are soft, cover with the cheesy egg mixture, and give the whole thing a good stir. Let cook on medium-low heat until the edges start to peel away from the pan, about 5-7 minutes. Transfer to the oven and bake for 10 minutes, or until the egg looks firm but still has a bit of a jiggle in the centre.
Remove and let sit at room temperature for 5 minutes. Cut into slices and serve with prosciutto slices.
