Position an oven rack 4 to 6 inches from the broiler and heat the broiler on high.
In a medium bowl, whisk together the eggs, milk, 1/4 cup of the Parmigiano, the mustard, 1/4 tsp. salt, and about 3 grinds of pepper. Set aside.
Heat the oil in an 8-inch nonstick skillet over medium heat. Add the diced pancetta. Cook, stirring constantly, until the pancetta is pale golden, 3 to 4 minutes. Add the shallot and asparagus and cook, stirring frequently, until the asparagus is crisp-tender, about 3 minutes.
Reduce the heat to low and pour the egg mixture into the skillet. Cook, giving the eggs a few stirs in the beginning to distribute the asparagus, until the eggs are well set and slightly puffed around the edges but are still loose in the center, 13 to 15 minutes.
Sprinkle the remaining 1 Tbs. Parmigiano on top of the frittata. Slide the pan under the broiler and cook until the frittata is puffed, browned, and set, 3 to 4 minutes. Let cool for 5 minutes (just enough time to toss together a salad). Cut into wedges and serve.