This burger won our praises because of its ridiculous factor. If you’re wondering why they named it the Ultimelt, maybe the fact that it’s sandwiched in between two gooey grilled cheeses will settle it for you. We don’t think it needs anymore explanation.
Mix all ingredients in a bowl until completely incorporated.
Heat a medium-sized skillet to heat. Butter 1 side of each slice of bread and add 2 slices, butter side down, into the skillet. Layer 2 slices of cheese on each piece of bread, and then top each grilled cheese with the other slices of bread, butter side up. Grill for 2 minutes on each side until the bread is golden brown and the cheese is melted.
Melt the butter in a large pot and add the sliced onions. Cook, on very low heat, until caramelized, about 30 minutes. When the onions are nearly done, add the sugar and vinegar and reduce until syrupy.
On a griddle of flat top, sear the burger to desired doneness, about 2 minutes per side for medium. When almost ready, add the Gruyère, caramelized onions, and bacon and cook until the cheese has melted. Place on 1 grilled cheese and add the Go Burger Sauce. Top with the other grilled cheese, cut on a bias, and serve.