Combine marinade ingredients in a zip-top plastic bag; add steak to the bag. Marinate 15 minutes to 2 hours for Sirloin or 2 hours to overnight for Flank steak.
Cook quinoa according to package instructions.
Combine Tzatziki sauce ingredients in a bowl. Set aside.
Heat a grill pan on the stove over MEDIUM-HIGH heat.
Remove steak from bag, discard leftover marinade. Once the grill pan is hot, place steak on the pan and grill for 10-12 minutes (medium rare internal temperature = 145°F; medium = 160°F). Let the steak rest for 3 minutes. Slice steak against the grain to serve. Salt to taste, if desired.
Evenly divide quinoa, spinach, tomatoes, and cucumbers in four bowls. Top bowls with feta cheese and 2 Tbsp. Tzatziki sauce.
Nutrition information per serving: 428 Calories; 168 Calories from fat; 18.8 g Total Fat (4.1 g Saturated Fat; 10.5 g Monounsaturated Fat); 63.4 mg Cholesterol; 269.1 mg Sodium; 31.8 g Total Carbohydrate; 4.0 g Dietary Fiber; 31.8 g Protein; 4.4 mg Iron; 690.1 mg Potassium; 0.2 mg Thiamin; 0.4 mg Riboflavin; 10.9 mg Niacin (NE); 0.8 mg Vitamin B6; 1.5 mcg Vitamin B12; 5.7 mg Zinc; 32.5 mcg Selenium; 121.9 mg Choline.
This recipe is an excellent source of Protein, Iron, and Thiamin, Riboflavin, Niacin, Vitamin B6, Vitamin B12, Zinc, Selenium, and Choline. It is a good source of Dietary Fiber and Potassium.