1. Preheat your grill to medium-high heat.
2. Remove both ends and outermost layers from the onions. Roughly chop and combine with a couple tablespoons of butter in a hot pan. Cover for 3-5 minutes, or until the onions become translucent and tender, and then remove the lid and reduce the heat to medium. Continue to cook the onions until they are caramelized.
3. Next, season the ground beef with salt and pepper and form into 12 equal patties. Press flat and grill to your preferred doneness.
4. Now, preheat a nonstick pan or griddle to medium heat for toasting the sandwiches; assemble the sandwiches while the pan warms. From the bottom up: a piece of bread, a slice of cheddar, a spoonful of sauteed onions, the burger, a slice of Swiss, and the final piece of bread.
5. Rub butter on the hot griddle and place the sandwiches on top. Toast until the bread is golden brown and the cheese has started to melt. Flip to finish cooking, and repeat until all of the sandwiches are toasted.
6. Slice in half and finish with a generous drizzle of secret sauce.