Rinse and pat dry your flank steak. Generously season both sides with kosher salt and pepper. Tenderize if desired.
Allow to rest for at least 30 minutes before serving. Make sure to grill the meat at room temperature, not cold.
In a medium bowl, whisk together the balsamic dressing ingredients. Set aside (this step can also be done a few days in advance).
Preheat your grill to 400 degrees, or medium-high heat. Grill your flank steak for 5-6 minutes each side for medium rare. When meat is cooked, remove from grill and allow to rest for at least 10 minutes. When the meat has rested, slice thinly, or cut into bite-size cubes.
Assemble your salad, layering the butter leaf, steak, avocado, strawberries, goat cheese, and pistachios. Drizzle the dressing on top and serve while the steak is warm.