Place the sirloin in the freezer for 20 to 30 minutes prior to slicing to make it easier to achieve very thin cuts.
In a large 14-inch sauté pan, heat 2 tablespoons of vegetable oil over medium-low heat. Add the sliced onion, red bell pepper, and green bell pepper. Sauté gently, stirring occasionally, until the vegetables are extremely tender. Lower the heat slightly if browning occurs. While the vegetables cook, begin preparing the beef.
Remove the partially frozen steak and trim any excess fat. Slice it as thinly as possible against the grain, then chop the slices finely into approximately ¼-inch pieces. Continue chopping until the pieces are uniformly small. Stir the vegetables intermittently as you work.
Once the vegetables are fully softened, transfer them to a separate dish. Return the pan to medium-high heat, add a bit more oil if needed, and cook half of the chopped steak, stirring frequently, for 4 to 5 minutes or until no pink remains. While cooking, season the beef with ¾ teaspoon of the seasoned salt and ½ teaspoon of the garlic pepper. Once done, remove the steak with a slotted spoon and place it with the sautéed vegetables.
Add more oil if necessary, and repeat the cooking process with the remaining steak, seasoning it with the remaining seasoned salt and garlic pepper. Transfer the second batch to the vegetable mixture when fully cooked.
Preheat the oven to 400°F. Carefully split open each hoagie roll without separating the halves. Generously spread the interior with softened butter and arrange them cut side up on a baking sheet. Toast in the oven for 3 to 5 minutes until golden and crisp.
Drain all excess juices from the sauté pan and return it to the stove over medium heat. Return all the cooked steak, onions, and peppers to the pan. Layer the provolone cheese slices evenly over the mixture in a circular pattern. Cover the pan and heat for 4 to 5 minutes, or until the cheese has fully melted. Stir the mixture to combine.
Using tongs, distribute the hot steak and vegetable filling evenly into the toasted hoagie rolls. Serve immediately.