Place potatoes in a large pot; add cold water to cover by 1 inch. Bring to a boil over high. Reduce heat to medium-high, and boil, stirring occasionally, until potatoes are very tender when pierced with a fork but not falling apart, 22 to 25 minutes. Drain well. Let stand until cool enough to handle, about 15 minutes. Rinse pot, and set aside.

Press potatoes through a potato ricer into a medium bow (or mash with a fork in a medium bowl until mostly smooth and fluffy); spread potatoes in an even layer on a clean work surface. Let cool almost to room temperature, about 10 minutes. Drizzle egg yolk evenly over potatoes. Sprinkle 1 teaspoon salt and 2/3 cup flour evenly over top. Fold mixture over onto itself several times using a bench scraper or rubber spatula. Sprinkle with remaining 2/3 cup flour, and gently knead just until all flour is incorporated and dough is soft and smooth. (To test the consistency, pinch off a small piece of dough, and roll it into a rope. If it falls apart, knead in 1 tablespoon additional flour, and test again.)

Line a large baking sheet with parchment paper, and sprinkle lightly with flour; set aside. Divide dough into 8 equal pieces. Working with 1 piece at a time (keeping remaining dough covered with a clean towel to prevent it from drying out), roll dough into a 1/2-inch-thick rope. Cut rope crosswise into 1/2-inch pieces. Place gnocchi in a single layer on floured baking sheet. Repeat with remaining dough pieces.

Fill cleaned pot with salted water, and bring to a boil over high. Lightly oil a rimmed baking sheet, and set aside. Working in 4 batches, add gnocchi to boiling water; stir once, and let cook, undisturbed, until gnocchi puff and float to the surface, about 1 minute. Continue to cook gnocchi 1 more minute, and transfer to oiled baking sheet using a slotted spoon. Repeat with remaining gnocchi. Reserve 1/2 cup cooking liquid.

Heat a large nonstick skillet over medium. Add pancetta, and cook, stirring occasionally, until browned and crisp, about 6 minutes. Transfer pancetta to a small bowl using a slotted spoon; reserve drippings in skillet.

Add oil to skillet with drippings, and heat over medium-high. Add onion; cook, stirring occasionally, until translucent, 3 to 4 minutes. Add asparagus, pepper, and remaining 1/2 teaspoon salt; cook, stirring occasionally, until asparagus is bright green, about 1 minute. Add gnocchi and 1/2 cup reserved pasta cooking liquid; cook, tossing constantly, until gnocchi is coated in a creamy sauce, 1 to 2 minutes. Remove from heat, and season with additional salt to taste. Top with crispy pancetta, and serve immediately.