Preheat the grill to medium-high heat
Season the flank steak generously with salt and pepper
Place the steak on the grill and cook for about 4 minutes per side or until it reaches your desired level of doneness
Remove the steak from the grill and let it rest for at least 5 minutes before slicing it thinly against the grain
While the steak is resting, prepare your salad by tossing mixed greens, red onion, cherry tomatoes, and cucumber in a large bowl
Add the dressing of your choice and toss again to coat evenly
Serve grilled flank steak salad topped with diced avocado