Place the onions face down on a cutting board and slice lengthwise as thinly as possible, creating half-moons. Alternatively, use a food processor with the slicing disc fitted (I use my 3 mm disc) to slice the onions.

In a large skillet or Dutch oven (choose a wide cooking vessel so that the onions caramelize and don’t steam) heat oil and butter over medium heat. Add onions and season with salt and optional sugar. For the first few minutes stir and flip often with a wooden spatula make sure that the same batch of onions isn’t always on the bottom.

After about five minutes, once the onions have started to sweat and soften, turn down the heat to medium low (you might even have to cook on low depending on the strengthen of your flame). Stir every 2-3 minutes to prevent onions from sticking and ensure that they cook evenly. As the onions starts to caramelize, be mindful of the heat, lowering if you feel that they are browning too quickly (you want them to brown slowly and not burn). If the bottom of your skillet starts to brown in spots, use the wooden spatula to scrape it clean (you also might rotate the skillet since the heat might not be distributing evenly). Keep cooking and stirring onions until they turn a blondish brown, about 15-20 more minutes. While you can cook your onions longer for a sweeter end result, this is how I like them (they are also perfect at this point for French Onion Soup; link to recipe in notes).