Steps
1. Preheat oven or air fryer to cook fries according to package instructions until crispy.
2. While fries cook, caramelize the onion in a pan over medium heat with a little oil until golden brown.
3. Mix mayonnaise and Dijon mustard in a small bowl.
4. Butter one side of each bread slice.
5. On the non-buttered side of one slice, spread the mayo-mustard sauce.
6. Layer the shredded beef evenly on top of the sauce, season with salt and pepper.
7. Add caramelized onions over the beef.
8. Once fries are cooked, place a generous handful over the onions.
9. Optionally, sprinkle shredded cheese over fries.
10. Top with the second slice of bread, buttered side up.
11. Toast the sandwich in a skillet over medium heat, pressing gently, about 3-4 minutes per side, until bread is golden and cheese (if used) melts.
12. Remove, slice in half, and serve immediately.