Heat oven to 350 degrees, if using oven cooking method.
Heat olive oil in a large ovenproof, nonstick skillet. Add pumpkin and cook over medium heat until almost tender. Add the scallions and cook for 1-1/2 minutes longer.
Add the prosciutto and cook, stirring occasionally, until slightly crisp, 3 to 4 minutes.
In a medium bowl, beat the eggs lightly. Whisk in the milk, Parmesan and black pepper.
Add the pumpkin cubes, scallions and prosciutto to the egg mixture. Pour into the ovenproof skillet and distribute evenly. The eggs should barely cover the prosciutto and vegetables.
Oven method of cooking:
Bake until browned around the edges and puffed, 15 to 20 minutes. A knife inserted in the middle should come out clean.
Stovetop method of cooking:
(No need to preheat the oven.)
Cover non-stick skillet and cook over very low heat, until the top of the tortilla is firm, about 15 minutes.
Turn a large plate upside down and place it over the skillet like a lid. Holding the plate in place, overturn the skillet, dropping the frittata onto the plate. Slide the frittata back into the skillet, and brown the other side for a few minutes.
Using the same technique, turn the frittata onto a large round plate to serve.
* For a rich, earthy flavor, substitute Fabrique Délices truffle butter for part or all of the olive oil.
Serve with an arugula and goat cheese salad.