Preheat oven to 450°F.
Wash, peel, and slice potatoes into ¼-inch thick strips. Place on a baking sheet and toss with oil and salt.
Spread potatoes out in a single layer and place in the oven (it doesn't have to be fully heated). Roast until golden brown, tossing once halfway through, 15-20 minutes.
Peel and thinly slice onion. Peel and mince garlic. Add both to a medium bowl.
Preheat a skillet over medium-high heat.
Thinly slice steak against the grain. Add steak and salt to the bowl with the onion and toss to combine.
When the skillet is hot, add oil and swirl to coat the bottom. Add steak mixture and cook, stirring often, until steak is browned and cooked to desired doneness, about 4-6 minutes.
Meanwhile, wash and dry bell pepper and mushrooms. On a clean cutting board, thinly slice the mushrooms and pepper.
When the steak is done, transfer to a medium bowl and cover with foil. Return skillet to medium heat; add oil, peppers, and mushrooms. Cook, stirring often, until veggies are browned, 5-7 minutes. Add veggies to the bowl with the steak.
In a small bowl, make a slurry by whisking broth with cornstarch until smooth.
Return skillet to medium-high heat; add the cornstarch slurry, additional broth, Worcestershire sauce, pepper, and any juices from the bowl with the steak. Cook, whisking constantly, until slightly thickened, 2-3 minutes.
Split buns in half and place on a baking sheet pan, cut-side facing up. Layer cheese over bottom buns. Place in the oven and toast until bread is golden brown, and cheese is melted, 2-3 minutes.
Divide steak and veggies between bottom buns, drizzle with warm sauce and top with the other half of the bun. Serve with fries and remaining sauce on the side for dipping. Enjoy!