Add 1-2 tablespoons of butter to a pan over medium heat. Caramelize your onions until soft, fragrant and beginning to char, about 10-15 minutes.
Once the onions are finished, turn off the heat and move them to one side of the pan. Add about a 1/4 lb of roast beef slices to the other side - just to warm them up a bit.
Liberally butter both pieces of a nice hearty bread. (Oat nut shown here.)
Heat a separate pan over medium heat, and place one slice of bread buttered side down.
Immediately add 2-3 slices of cheese. (Shown here a combination of Provolone and American.)
Top with warmed Roast Beef, desired amount of caramelized red onion (about 1/4 cup) and another 3 slices of cheese.
Place the other piece of bread on top (butter side up).
After about 4 minutes - check the bottom piece of bread by lifting the corner with a spatula. If browned to your liking, carefully flip and grill the other side, about 4-5 minutes. (You may want to cover the pan to help promote cheese melting.)
Eat immediately OR you can keep it warm in a 200 degree oven for a few minutes until ready to eat or serve.