Pulse 1/4 cup blueberries, 1/4 cup walnuts, vinegar, shallot, sugar and 1/4 teaspoon salt in a food processor to form a chunky paste. With the motor running, add oil until incorporated. Transfer the dressing to a large bowl.
Sprinkle steak with pepper and the remaining 1/4 teaspoon salt. Oil the grill rack (see Tips). Grill the steak about 5 minutes per side for medium-rare, 6 minutes per side for medium. Let rest on a clean cutting board for 5 minutes.
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.
TIPS & NOTES
Tips: To toast chopped nuts, place in a small dry skillet and cook over medium-low heat, stirring constantly, until fragrant and lightly browned, 2 to 4 minutes.
To oil a grill rack, oil a folded paper towel, hold it with tongs and rub it over the rack. (Do not use cooking spray on a hot grill.)
Per serving: 392 calories; 26 g fat (5 g sat, 7 g mono); 68 mg cholesterol; 11 g carbohydrates; 1 g added sugars; 29 g protein; 3 g fiber; 494 mg sodium; 748 mg potassium.
Nutrition Bonus: Vitamin A (114% daily value), Zinc (36% dv), Vitamin C (35% dv), Folate (34% dv), Magnesium (26% dv), Iron & Potassium (21% dv)