With the rack in the middle position, preheat the oven to 400°F (200°C).
In a large non-stick skillet over medium-high heat, brown the bell pepper and onion in half of the oil. Stir in the chipotle sauce. Season with salt and pepper. Set aside on a plate.
In the same skillet, brown half of the beef at a time in the remaining oil. Coarsely chop the beef.
On a work surface, cut the buns in half horizontally without going all the way through to the other side. Open them like a book and place on a non-stick baking sheet. Layer in the cheese, meat and vegetables.
Bake for 5 minutes or until the cheese has melted. Garnish with quick-pickled cucumbers, if desired.