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