Adjust oven rack 6 inches from the heating element. Preheat broiler on low. Cut each baguette piece in half crosswise. Scoop out some of the bread from the center of each, leaving thick walls. Place bread cut side up on a baking sheet.

Season steak with the garlic powder, 1/2 teaspoon kosher salt, and 1/4 teaspoon freshly cracked black pepper. Heat a large skillet over medium-high. Coat with olive oil cooking spray and add half of the steak, making sure not to crowd the pan. Cook 1 minute until browned, then turn and cook an additional 30 seconds. Set aside in a large dish. Repeat with the remaining steak. Wipe skillet clean.

Reduce heat to medium, add 1 teaspoon olive oil to skillet; add onion and bell pepper and season with a pinch of salt and black pepper. Cook 2 to 3 minutes, or until vegetables are soft and slightly browned. Add to steak.

Add an additional 1/2 teaspoon olive oil to skillet. Add mushrooms; season with a pinch of salt. Cook, stirring, 3 to 4 minutes or until soft. Remove from heat; stir in steak, onions, and peppers.

Mound steak and vegetables evenly onto bread and top with cheese. Heat under the broiler until cheese melts, about 2 minutes. Serves 4.