1. First, caramelize the onion (you can do this in advance, if desired): In a large skillet, heat half of the oil or butter over medium-high heat. Add the sliced onion and sauté, stirring, until beginning to brown, about 5 minutes. Lower the heat and continue cooking, stirring, until the onion is very brown and caramelized. Transfer the caramelized onion to a plate and set aside.

2. Next, make the aioli: In a small bowl, mix together the mayonnaise with the seasoning salt to taste. Set aside.

3. In the skillet from cooking the onion, cook the bacon until crispy. Drain the bacon on paper towels and set aside.

4. Spread 1 side of each bread slice with the aioli. On 1 piece of bread, layer the cheeses. On the other piece of bread, layer the caramelized onion, then the sliced avocado, then the bacon.

5. Scramble the eggs last: Heat a flat-top griddle over medium-high heat, then add the remaining oil or butter. Add the eggs and season with kosher salt and black pepper to taste, then scramble them to your liking (I like mine soft-scrambled). Add the eggs on top of the cheeses, then close the two halves of the sandwich.

6. Add the assembled sandwich to the griddle and cook until browned, 3–5 minutes per side. Cut in half and serve!