Preheat oven to 425 degrees. Place the trimmed asparagus on a parchment lined sheet pan. Drizzle evenly with 2 tablespoon of olive oil and season with salt and pepper. Toss the asparagus spears to evenly coat and arrange them in a single layer. Bake at 425 for 10-15 minutes or until the tips of the spears begin to brown and crisp and the stalks are tender. If you have mature asparagus with thicker spears this may take a few minutes longer.
While the asparagus is in the oven, prepare the vinaigrette. In a medium skillet over medium heat, cook the prosciutto until light golden brown, about 5 minutes. Remove the prosciutto from the pan to drain on a paper towel reserving the drippings. You should have about 2 Tbsp. of drippings left. Add the onion to the reserved drippings and sauté for 2 minutes. Add the garlic and cook for another minute. Remove from the heat and whisk in the Dijon mustard and the vinegar. Finally, whisk in 3 Tbsp. of olive oil, taste and season with salt and pepper as desired.
To serve, place the roasted asparagus on a serving plate or platter, drizzle with the warm vinaigrette, sprinkle the cooked prosciutto over the asparagus and top with shaved Parmesan.
Note: Bacon or pancetta can be substituted for the Prosciutto. This recipe serves 3-4 depending on the size of the asparagus. If you have any dressing left over, try it on a spinach salad!