The first step is to cut the bacon crosswise into one-inch pieces and cook them in a large skillet, stirring occasionally until the fat is rendered and the bacon is lightly browned about 20 minutes.
Carefully transfer bacon to a sieve so the fat can drain off. Reserve one tablespoon of the bacon fat.
Chop two large onions. Add the tablespoon of bacon fat, chopped onions, and garlic to the hot skillet. Allow this to cook until the onions are translucent, about 7 minutes.
Add the strong brewed coffee, balsamic (or apple cider —see note 2) vinegar, brown sugar, sea salt, molasses, and maple syrup and bring to a boil; cook stirring and scraping up any browned bits from the skillet, for about 2 minutes.
Add the cooled bacon, stir to combine, reduce the heat to a bare simmer and cook uncovered, until the liquid is almost completely evaporated and turns syrupy, about 2 hours. OR transfer to slow cooker and cook on High for 3 to 4 hours; Low for 6 to 7 hours.
Once this bacon concoction has cooled slightly, hit it with an immersion blender (transferring to a food processor then pulsing until coarsely chopped works as well) and pulse just to smooth it out a bit—relish consistency.
Spoon into sterilized jars.