In a large skillet over mediumish heat, cook the bacon, stirring occasionally, until the fat is rendered and the bacon is lightly browned, about 20 minutes. Using a slotted spoon, transfer the bacon to paper towel-lined plates to drain.
Pour off all but 1 tablespoon drippings from the skillet and reserve for another use. Add the onions and garlic to the skillet and cook until the onions are translucent, about 6 minutes. Add the coffee, vinegar, brown sugar, and maple syrup and bring to a boil. Cook, stirring and scraping up any browned bits from the skillet, for 2 minutes. Add the bacon and stir to combine.
If making this on a stove top, reduce the heat to a bare simmer and cook uncovered, stirring occasionally, until the liquid almost completely evaporates and turns syrupy, 1 to 1 1/2 hours.
If making this in a slow-cooker, transfer the mixture to a 6-quart slow-cooker and cook on high, uncovered, until the liquid almost completely evaporates and turns syrupy, 3 1/2 to 4 hours.
Let the bacon concoction cool slightly before transferring it to a food processor and pulsing until coarsely chopped. Spoon the bacon lusciousness into individual jars or other resealable containers and refrigerate for up to 4 weeks. Transfer to a pan and rewarm gently over low heat prior to indulging.