Marinate the steak: combine garlic powder and smoked paprika. Coat flank steak or sirloin with olive oil, then sprinkle the spice mixture evenly. Let marinate for at least 10 minutes.

Cook vegetables: heat a skillet over medium heat and add olive oil. Sauté sliced onions and green bell peppers for about 7-8 minutes until soft and caramelized. In the last minute, add minced garlic.

Cook the steak: in the same skillet, cook the marinated steak for about 3-4 minutes per side for medium-rare doneness. Remove from heat and let rest for 5 minutes.

Assemble the sandwich: slice the rested steak against the grain. Mix mayonnaise and Dijon mustard in a bowl and spread on rolls. Layer sliced steak, sautéed vegetables, and cheese.

Melt the cheese: broil sandwiches for 2-3 minutes until cheese is bubbly, or toast in a skillet for a crispy exterior.

Add greens: top with fresh arugula or lettuce, close the rolls, and cut in half to serve immediately.