This is a different sort of frittata, not the neat golden round of well-set eggs that’s probably most familiar. Here the eggs are in the skillet for barely a minute, just long enough to gather in soft, loose folds, filled with morsels of asparagus and shreds of prosciutto.
Snap off the tough bottom stubs of the asparagus, peel the bottom few inches of each spear, and cut them crosswise in 1 1/2-inch pieces. Slice prosciutto or bacon into strips, or lardoons, about 1 inch long and 1/3 inch wide. Trim the scallions, and cut crosswise into 1-inch pieces.
Pour the olive oil into the skillet, scatter in the lardoons, and set over medium heat. When the strips are sizzling and rendering fat, toss in the cut asparagus, and roll and toss them over a few times. Cover the skillet, and cook, still over moderate heat, shaking the pan occasionally, until the asparagus is slightly softened, 5 minutes or so.
Scatter the scallion pieces in the pan, season with a couple pinches of salt, and toss the vegetables and lardoons together. Cover the skillet, and cook, shaking the pan and stirring occasionally, until the scallions and asparagus are soft and moist, 7 or 8 minutes more. Meanwhile, beat the eggs thoroughly with the remaining salt and generous grinds of black pepper.
When the vegetables are steaming in their moisture, uncover the skillet, raise the heat, and cook, tossing, for a minute or so, until the water has evaporated and the asparagus and scallions seem about to colour.
Quickly spread them out in the pan, and pour the eggs over at once. Immediately begin folding the eggs over with the spatula, clearing the sides and skillet bottom continuously, so the eggs flow and coagulate around the vegetables and lardoons.
When all the eggs are cooked in big soft curds, in barely a minute, take the skillet off the heat. Tumble the frittata over a few more times to keep it loose and moist. Spoon portions onto warm plates, and serve hot and steaming.