The first you need to make or acquire some bacon onion jam.
I have had some good commercial bacon jams, but nothing beats the homemade stuff. You can make it using my Bacon Onion Jam Recipe.
Next you need to butter 1 side of a thick slice bread. I like to use thick sliced Texas toast. I find that it is soft but yet holds up to frying on a flat top. I like to use either whipped butter or vegetable spread. That way you don't rip up the bread. I find that I get a better browning of the bread when I use the vegetable spread.
Next preheat your griddle to 350 degrees Fahrenheit or preheat a large pan on the stove over medium heat.
Once the griddle is heated. Place a slice butter side down onto the griddle. Then add 2 tablespoons of shredded sharp cheddar and 2 tablespoons of mozzarella cheese. Top with 3 tablespoons of Bacon Onion Jam.
Finally top with 2 more tablespoons of sharp cheddar cheese and 2 tablespoons of mozzarella cheese.
Add another piece of buttered bread butter side up.
Grill for 2-3 minutes or until it is nicely browned. Flip and then grill for another 2-3 minutes or until the bread is nicely browned and the cheese is melted.