Add the butter and oil to a large, heavy-bottomed pot with a lid over medium heat (I use a 4-quart braising pan). Once the butter is melted, add the sliced onions, stirring to coat. Cover the pot. Cook 12-15 minutes, until softened and fragrant. The onions shouldn’t require too much attention at this point, but feel free to uncover and stir occasionally.
Once softened, uncover the pot and reduce heat to medium-low. Season the onions with 2 teaspoons kosher salt, stirring to combine. Continue to cook, stirring occasionally, until tender, fragrant, and deeply browned, 30-45 minutes more. See the Recipe Notes, below, for a few more helpful pointers.
Once the onions are caramelized, deglaze the skillet. Slowly pour in the dry sherry, using a wooden spoon to scrape up any browned bits formed on the bottom of the pan. Increase the heat to medium-high to bring the wine to a simmer, then simmer 2-3 minutes or until reduced. Remove from the heat.