Heat oil in a large cast-iron skillet over medium heat. Add pepper-and-onion mixture and cook until heated through, about 5 minutes. Add thinly sliced beef and cook with the pepper mix another 5 minutes Set aside and keep warm.
For cheese sauce, melt butter in a medium saucepan over medium heat. Add flour, whisking constantly until bubbly, about 1 minute. Slowly whisk in milk until thickened, about 3 minutes. Stir in black pepper, cheese, chipotle peppers, and adobo sauce.
Divide steaks and pepper-and-onion mixture among 4 baguette slices. Top with cheese sauce and remaining 4 bread slices.