Season the flank steak with salt and pepper.
Preheat a grill or skillet over medium-high heat.
Cook the steak for 4-5 minutes on each side for medium-rare.
Remove the steak and let it rest for 5 minutes before slicing.
In the same pan, sauté onions and mushrooms until soft.
Toast the bread lightly on the grill or in a toaster.
Spread mayonnaise on one side of each bread slice.
Layer lettuce on the bottom slice of bread.
Add sliced steak, sautéed onions, and mushrooms.
Top with provolone cheese and cover with the second slice of bread.