You have two schools of thought when it comes to caramelizing onions. The first, which harkens back to Julia Child and her superb soupe a l'oignon gratinee, or French onion soup, says true caramelization of onions takes about 40 minutes. The other holds that you can get splendidly sweet, caramelized onions in about 10 minutes. Both are correct, and you get excellent results when you combine both techniques. Start with fat, such as rendered bacon, use fresh, juicy onions, add a touch of sugar to catalyze the caramelization process, apply heat and you'll have rich, golden-brown onions Julia herself would claim.
Slice the bacon into 1-inch pieces. Use slab bacon if you want small, meaty bits to add to the caramelized onions later, and use regular, sliced bacon if you only want the bacon for the fat.
Lightly butter or oil a stainless-steel or cast-iron skillet and place it on the stove over low heat. Heat the pan for about five minutes and add the bacon in an even layer.
Cook the bacon until the fat melts away, fishing out the meaty pieces with a slotted spoon as needed. Drain the bacon pieces on a paper towel.
Pour the rendered bacon fat through a cheesecloth-lined sieve and into a heat-proof bowl or measuring cup. The cheesecloth traps the small browned pieces that burn when you caramelize the onions.
Return the pan to the stove and add a few tablespoons of rendered bacon fat. Set the heat to medium high and let the pan heat for about five minutes.
Add the sliced onions to the pan in an even layer, spreading them out, as needed, with a wooden spoon. Sprinkle a pinch of sugar over the onions and season with kosher salt. The extra touch of sugar catalyzes the caramelization process and helps the onions release their natural sugars.
Cover the pan with a lid to allow steam to build and help break down the onions. Cook the onions for about 2 1/2 to 3 minutes without disturbing them. The onions won't burn.
Uncover the onions and stir. Pour a tablespoon or two of water in the pan to deglaze it and scrape the fond, or caramelized bites of onions, from the bottom of the pan.
Cook the onions another five minutes, stirring occasionally. Deglaze the pan with water again when the fond builds up on the bottom.
Cook the onions until the water you deglazed with evaporates and the onions start sizzling again, about 30 seconds. The onions will have a deep, caramelized brown color and no raw onion taste. Garnish the onions with reserved bits of bacon, if desired.