For the bacon jam:In a medium pot, cook the bacon until nicely browned. Remove from pot, saving about 2 tbsp bacon fat. (Strain the rest off of the bacon)
In remaining bacon fat, cook the onions over medium heat until translucent. Add garlic and cook about a minute until fragrant.
Add bacon back to the pot. Add remaining ingredients.
Cook over medium heat, stirring occasionally, for about 30-40 minutes until thickened and syrupy (there shouldn't be a lot of liquid remaining).
Optional: Puree some or all of the jam. I pureed about ½ cup in the Magic Bullet, just to give it more of a jam consistency.
Serve warm or cold.
For the grilled cheese:Butter two slices of bread. Place one slice of bread, butter side down, into a pan preheated over medium heat. Top with cheese, bacon jam, and more cheese.
Top with second slice of buttered bread, butter side up.
Cook until golden brown. Flip and cook until golden brown.