Light a grill for medium-high, direct-heat grilling.
Combine the extra virgin olive oil, vinegar, shallot, and rosemary in a small bowl and season with salt and pepper. Set aside.
Season the steak with salt and pepper. Separately, rub the nectarine pieces with oil and season with salt and pepper.
Grill the steak until lightly charred outside and pink in the middle, for medium doneness (or until desired degree of doneness), 6 minutes per side. Transfer to a cutting board and let rest 5 minutes.
While the steak rests, place the nectarines on the grill cut-side down and grill until char marks appear, about 3 minutes. Transfer to another cutting board. (Keep the fruit separate from the resting steak to prevent meat juices from dampening it.)
Cut the steak across the grain into thin slices. Cut the nectarine sections into bite-sized chunks.
Place the lettuce in a bowl and toss with 2 tablespoons of the vinaigrette. Divide among 4 plates and top each evenly with steak, nectarines, blue cheese, toasted walnuts, and red onion. Drizzle with the remaining vinaigrette and serve.
Main-course salads are a great way to combine the hearty, juicy flavor of grilled steak with the tasty, nutritious goodness of fresh vegetables, fruit, and nuts—all in one bowl. Flank steak is one of the leanest cuts you can buy—and one of the best on the grill. And if you’ve never grilled fruit, now is the time to do it. Large pieces of sweet nectarine take on a slightly smoky taste with appealing char marks (just like meat), and they’re done in about three minutes.
Toast the walnuts in a small, dry skillet to bring out their deepest nutty flavor. Shake the pan frequently and toast about two minutes.
Now it is time to fire up your grill.
Slice the steak into small strips.
With four ounces of beef per serving, this simple and satisfying dish is nothing to feel guilty about. Ask your butcher to cut a small flank steak for you, or substitute two 8-oz boneless strip or rib-eye steaks. Be sure to trim the fat before slicing.