Pour 1/3 of vinaigrette into a resealable bag, and add steak. Seal bag, tossing to coat steak. Remove steak from bag and grill 5 minutes each side, or until medium-rare.
Transfer steak to cutting board and let rest 10 minutes. Thinly slice steak across the grain on a slight diagonal. Set aside.
In a large skillet, heat oil over medium-high heat. Add mushrooms, garlic, thyme, and pepper, and sauté until mushrooms are golden, about 5 minutes.
In a large salad bowl, add remaining vinaigrette, mushroom mixture, and spinach. Toss to coat. Top with sliced steak and onions.
Nutrition Info:
383 calories (31g protein, 13g carbohydrate, 23g fat) per serving