Bring a large pot of well-salted water to a boil. Meanwhile, put 2 Tbs. of the oil and the pancetta in a 12-inch skillet or sauté pan and cook over medium heat, stirring, until the pancetta begins to brown and renders much of its fat, 5 to 7 min. Using a slotted spoon, transfer the pancetta to a plate lined with paper towels. Raise the heat to medium high, add the asparagus, and season with 1/4 tsp. salt. Stir often until the asparagus starts to brown, about 3 min. Add the caramelized onions and chicken broth, reduce the heat to medium low, cover, and cook until the asparagus is tender, about 4 min. Stir in 1/4 tsp. pepper and set aside in a warm spot.

Cook the pasta in the boiling water, stirring often, until it’s just tender, about 11 min. Reserve 1/4 cup of the pasta water. Drain the pasta and add it to the pan with the asparagus. Set the pan over medium-high heat and toss well. Add 1/2 cup of the Parmigiano, the vinegar, and the cooked pancetta and continue cooking, stirring, for 1 min. to meld all the flavors. If the pasta begins to dry, add the reserved pasta water. Serve with a drizzle of the remaining 1 Tbs. olive oil and a sprinkling of the remaining Parmigiano.