Heat a pan on low-medium heat. Melt about a tablespoon of butter in the pan. Add the chopped yellow onions to the pan. Cook until the onions soften. Set aside.
Heat a pan to medium heat. Add a teaspoon of butter. Add two to four slices of bread at a time (depending on the size of your pan. Cook until the bread just starts to firm up. Flip the bread over.
Lower the heat to low medium. Spread some of the spicy mayo on each slice of bread.
Add cheese to each slice of bread. Cover the pan for about 30 seconds to encourage the cheese to start melting.
Add a tablespoon or two of the onions on one slice. Top that slice of bread with a handful of the shredded baked chicken. Top that same slice with a drizzle of the spicy mayo. Sandwich the slice of bread with the cheese on top of the bread with all of the other toppings.