Melt butter in a small skillet over medium-high heat. Add onions and cook until onions are soft and starting to caramelize, about 5 minutes. Remove from heat and set aside.
In a large bowl combine ground beef and Sriracha sauce. Mix well unto Sriracha is evenly distributed. Form mixture into two patties and season with salt and pepper.
Heat a large cast iron skillet over medium-high heat. Add hamburger patties and cook 5 minutes or until a crust forms on the bottom. Flip patties over and cook 3 minutes.
Using a wide spatula, break patties up into large pieces. Sprinkle cooked onion in between the spaces of the hamburger pieces. Layer cheese over the beef, and cook 2 minutes or until cheese is melted.
Divide meat mixture evenly between the toasted hoagie rolls. Top each with shredded lettuce and sliced tomato.
