01 -  Heat olive oil in a large skillet over medium heat. Add sliced onions and bell pepper; sauté for 5 to 7 minutes, stirring occasionally, until vegetables are softened and lightly caramelized. Transfer to a plate and set aside. 
 02 -  Increase heat to medium-high. Add chopped steak to the same skillet and season with salt and pepper. Cook for 2 to 3 minutes, stirring until just browned. Return onions and peppers to the pan, toss to combine, then remove from heat. 
 03 -  Spread softened butter onto one side of each bread slice. Arrange 4 slices, buttered side down, on a clean work surface. Top each with 1 slice of provolone cheese, a generous portion of steak mixture, another slice of provolone, then finish with remaining bread slices, buttered side up. 
 04 -  Heat a clean skillet or griddle over medium heat. Grill sandwiches in batches, gently pressing with a spatula, for 3 to 4 minutes per side until the bread is golden brown and cheese is melted. 
 05 -  Remove sandwiches from the skillet, allow to rest for 1 minute, then slice and serve while hot.