Place steaks in a shallow baking dish and cover with the steak sauce, let marinate for 2 hours, turning once. Preheat grill. Remove steaks from marinade and season with salt and pepper to taste. Grill for 3 to 4 minutes on each side for medium-rare . Remove and let rest. Brush onion slices and tomatoes with olive oil and season with salt and pepper to taste. Grill on both sides until just cooked through. Place spinach in a large bowl and toss with balsamic vinegar, olive oil and salt and pepper to taste. Divide the spinach among 4 large salad dishes or platters. Slice each steak into 1/2-inch thick slices and fan on top of the spinach. Garnish with grilled tomatoes and onions and drizzle with Poblano Vinaigrette.
Mesa Steak Sauce:
Mix together in a small bowl until combined. Season with salt and pepper to taste.
Combine poblanos, onion and lime juice in a blender and blend until smooth. While the blender is running, add the oil slowly until emulsified. Add the spinach and blend until smoooth. Add the honey and season to taste with salt and pepper.