Prep: 1. Preheat grill or broiler. 2. Sprinkle steak with pepper and salt to taste. Make: 1. Pulse 1/4 cup blueberries, walnuts, vinegar, sugar (if using) and 1/4 teaspoon salt in a food processor or blender. Slowly pour in oil, whisking or blending constantly. 2. Grill or broil the steak about 4-5 minutes per side for medium-rare, 6 minutes per side for medium (depending on thickness). Remove from heat, cover with foil and let rest for 5 minutes before slicing. 3. Add spinach to the bowl with the dressing; toss to coat. Divide the spinach among 4 plates. Thinly slice the steak crosswise. Top the spinach with the steak, feta and the remaining blueberries and walnuts.
This is also great with feta cheese crumbled on top.