Preheat the oven to broil. Have nearby a large sheet pan lightly spritzed with cooking spray.
Season the cubed steak on both sides with steak seasoning. Adjust the amount to your taste. Sprinkle each piece on both sides with a few dashes of Teriyaki sauce.
Heat a few drizzles of olive oil in a large grill pan or skillet. Add the seasoned steak to the skillet.
Brown on both sides, cooking for around 2 minutes each side, depending on thickness then remove to the baking sheet.
Add the green pepper and onion to the skillet. Season with garlic salt and red pepper flakes. Adjust the amount to your taste. Cook until beginning to soften and brown leaving crisp tender.
Top the steaks evenly with the green pepper and onion then cover each steak with 2 pieces of mozzarella cheese.
Place into the oven under the broiler until the cheese is bubbly, melted and browned.
Rest for 5 minutes then serve on toasted garlic butter hoagie rolls.