Caramelized Onion and Sage Dip

6 tablespoons unsalted butter 24 whole sage leaves (from about 4 large sprigs; 1/3 cup) 2 large onions, halved and thinly sliced (about 8 cups) Kosher salt and freshly ground black pepper 1 cup sour cream 8 ounces cream cheese, at room temperature Potato chips, for serving