Cut 1 ½ pounds yellow onions into paper-thin slices (or use a mandoline). Heat 3 tablespoons olive oil in a large pot over medium-high heat until shimmering. Add the onions and cook, stirring constantly, until dark and caramelized, 15 to 20 minutes. Add 12 ounces dried orecchiette, 1 (32-ounce) box beef broth, ⅔ cup water, and ⅓ cup ruby port, and cook until the liquid is fully absorbed, about 12 minutes. Meanwhile, grate 2 ounces Gruyère cheese (about ¾ cup).
Turn off the heat and stir in the cheese. Taste and season with kosher salt and freshly ground black pepper as needed.
