If you’re using the bacon, fry it until browned and nearly crisp, then scoop it out to drain on a paper towel. Throw out the bacon grease, wipe out the pan, and add the butter. When melted, add the onions, thyme, and ¾ teaspoon salt. Cook over medium heat, stirring occasionally, about 25 minutes in all. At first the onions will be very moist, but after 10 minutes their water will have cooked off and they’ll begin to color. They needn’t be caramelized, but just take on a faint golden hue. When done, let them cool slightly. Taste for salt—they’ll be very sweet, so you might want to add more—and season well with pepper.