- Blend 2 Tbsp. less sodium soy sauce, 1 Tbsp. vinegar, garlic and pepper; pour over steak in large plastic food storage bag. Press air out of bag; close top securely. Turn bag over several times to coat both sides. Marinate 30 minutes; turn bag over occasionally.
- Meanwhile, combine remaining 1 Tbsp. less sodium soy sauce, 2 Tbsp. vinegar, oil and oregano.
- Broil steak 5 minutes on each side (for medium-rare), or to desired degree of doneness.
- Cut steak across grain into thin slices; combine with spinach, mushrooms and oranges in large bowl.
- Blend soy sauce mixture; pour over spinach mixture and toss to combine.