In a medium sauce pan on medium-low heat, cook bacon cubes until they are releasing a lot of the grease, about 10 to 15 minutes. Add chopped onion, garlic, pepper and mustard seeds. Cook for about 10 minutes until onion starts to turn translucent and turn down to low.
Simmer for 1 to 2 hours, stirring occasionally.
Add apple cider vinegar and brown sugar and cook until incorporated.
Remove bay leaf and serve warm or add to a jar and use cold.