Heat wide-bottomed skillet on medium-high heat and add oil and butter.Once oil is shimmering, add sliced onions.Sprinkle salt over onions and stir until onions are lightly coated with oil.Stir every 5-10 minutes, flipping the bottom onions to the top and scraping the bottom of the pan.After initial ten minutes or so of sweating, turn heat down to medium.After about thirty minutes, if onions start to dry out, you can add a tad more oil or a little water, but only a tablespoon or so.If sticking too bad, turn heat down to low.Cook until amber and soft, about an hour.Store in refrigerator for one week or in freezer for a few months.