Chop bacon into bite-sized pieces and place in a large skillet over medium heat. Cook for 5 to 7 minutes, stirring occasionally, until bacon is crispy and fat is rendered. Leave about 2 tablespoons of bacon fat in the skillet.

Use a slotted spoon to transfer cooked bacon onto a paper towel-lined plate to drain. Leave bacon fat in the skillet, pouring out any excess if more than 3 tablespoons.

Cut onions in half from root to tip, then slice thinly into half-moon shapes to yield about 6 cups of sliced onions.

Add sliced onions to skillet with bacon fat and reduce heat to medium-low. Cook onions slowly for 15 to 20 minutes, stirring occasionally, until soft and golden brown, allowing moisture to evaporate.

Sprinkle brown sugar, salt, black pepper, and garlic powder over the onions. Stir well to dissolve the sugar and coat the onions evenly.

Pour in balsamic vinegar and stir again, allowing the vinegar to bubble slightly on contact.

Return the cooked bacon pieces to the skillet and stir thoroughly to combine.

Continue cooking over medium-low heat for another 10 to 15 minutes, stirring frequently to prevent sticking, until the mixture thickens to a jam-like consistency and deepens in color.

Remove skillet from heat and let the jam cool for at least 15 minutes.

Transfer the cooled jam into sterilized glass jars and refrigerate. The jam will thicken slightly more as it cools.