Cook the bacon. Place the bacon in a 3- to 4-quart Dutch oven or heavy-bottomed saucepan over medium heat and cook, stirring frequently, until the fat is rendered and bacon is browned, about 30 minutes. Transfer cooked bacon with a slotted spoon to a double layer of paper towels. Drain all but 2 tablespoons of the bacon fat from the pan.

Caramelize the onions. Add the onions to the Dutch oven and lower the heat to medium-low. Cook, stirring occasionally and scraping any browned bits from the bottom of the pan, until the onions are very soft, deeply browned, and significantly reduced in volume, 30 to 40 minutes.

Deglaze and add seasonings. Add the bourbon and scrape up any browned bits stuck to the bottom of the pan. Increase the heat to medium, and add the maple syrup, vinegar, and pepper. Bring to a simmer and cook until thick and syrupy, 3 to 5 minutes.

Pulse in a food processor. Transfer the mixture to the bowl of a food processor fitted with the blade attachment. Pulse to a spreadable, but chunky, consistency, about 5 pulses. If not eating immediately, transfer to 1 (8-ounce) or 2 (4-ounce) glass jars. Cool to room temperature, then refrigerate.