Cooking spray 1 (12-oz.) flank steak 3/4 teaspoon kosher salt, divided 1/2 teaspoon black pepper, divided 2 teaspoons olive oil 3/4 cup chopped white onion 1/2 cup unsalted chicken stock (such as Swanson) 1 (15-oz.) can unsalted pinto beans, rinsed and drained 3 cups chopped romaine lettuce 1 1/2 cups chopped tomato 1 ounce tortilla chips 1 ripe avocado, thinly sliced 1 1/2 ounces queso fresco, crumbled (about 1/3 cup) 4 lime wedges