Combine marinade ingredients in a small bowl. Place half of the marinade in a zip-top plastic bag; reserving the other half for basting. Add Greek seasoning and steak to bag. Marinate 15 minutes to 2 hours for Sirloin or 2 hours to overnight for Flank steak.
Heat a grill pan on stove on MEDIUM HIGH heat.
Remove steak from bag, discard leftover marinade. Once grill pan is hot, place steak on pan and grill 10-12 minutes for medium rare (145°F) to medium (160°F) doneness, turning occasionally and basting with reserved marinade during the last 5 minutes. Remove steak from pan, and rest for 3 minutes. Salt to taste.
Steam spinach in a pot and cook quinoa according to package instructions. Set aside.
Combine Tzatziki sauce ingredients in a bowl.
Evenly divide quinoa, spinach, tomatoes, and cucumbers in four bowls. Top bowls with feta cheese and 2 Tbsp. Tzatziki sauce.
Suggested toppings: pistachios and pomegranate reduction
Nutrition information per serving: Calories 440; Total fat 14 g (Sat. fat 3.5g; Trans fat 0g); Cholest. 64mg; Sodium 430mg; Total Carb. 42g; Fiber 10g; Total Sugars 4g; Protein 37g; Vit D (0% DV); Calcium (25% DV); Iron (60% DV); Potas. (15%DV)