1 large yellow bell pepper, cut into 1/2-inch-wide strips
1 large red bell pepper, cut into 1/2-inch-wide strips
2 poblano peppers, seeded and cut into 1/2-inch-wide strips
6 hoagie or steak rolls, split but not sliced all the way through
12 slices pepper Jack or provolone cheese
Freeze the steaks for 10 minutes to make them easier to slice. Remove the steaks from the freezer and slice against the grain into long, 1/4-inch-wide strips.
In a large skillet, heat 1 tablespoon of the oil over high heat. Add half the sliced steak, season with 1/2 teaspoon of the fajita seasoning, and sear, stirring occasionally, until browned, 3 to 4 minutes. Transfer the steak to a plate and cover with foil. Repeat with 1 tablespoon of the oil, the remaining steak, and 1/2 teaspoon of the fajita seasoning. Reserve with the rest of the steak.
Reduce the heat under the skillet to medium-high. Add the butter and remaining 1 tablespoon oil and heat until the butter bubbles. Add the onion, salt, and black pepper and sauté until the onion is softened and translucent, 4 to 5 minutes.
Add the bell peppers, poblano peppers, and the remaining 1 teaspoon fajita seasoning and sauté, stirring often, until the onions and peppers are caramelized, 15 to 20 minutes.
Position a rack 4 to 5 inches from the heat and turn the broiler to high. Line a sheet pan with foil.
Add the steak to the skillet, toss to combine, and heat through, about 5 minutes. Remove from the heat.
Open the rolls and set them on the lined sheet pan. Using tongs, divide the steak and pepper mixture among the rolls and cover the steak with 2 slices of cheese per open-faced sandwich.
Broil the sandwiches until the cheese begins to bubble, 2 to 3 minutes.