1. Preheat grill to 350° to 400° (medium-high) heat. Sauté 1 thinly sliced small onion in 1/2 cup hot olive oil in a large skillet over medium-high heat 3 to 4 minutes or until golden and tender. Add walnuts and garlic; cook, stirring occasionally, 3 to 4 minutes or until walnuts just begin to turn golden brown. Remove from heat, and cool 5 minutes.
2. Rub steak with 1 Tbsp. olive oil, steak seasoning, and 1/2 tsp. salt. Toss 1/2-inch onion slices with remaining 1 Tbsp. olive oil.
3. Grill steak, covered with grill lid, 5 to 7 minutes on each side or to desired degree of doneness. At the same time, grill thick onion slices, covered with grill lid, 4 to 5 minutes on each side or until golden brown. Remove steak and onions from grill, and let stand 5 minutes. Thinly slice steak. Reserve half of steak and onions for Steak Wraps.
4. Whisk together vinegar, brown sugar, and mustard in a medium bowl. Gradually whisk in walnut mixture. Add 1/4 tsp. pepper and remaining 1/2 tsp.salt. Reserve 1/2 cup vinaigrette for Steak Wraps.
5. Toss lettuce and grilled onions with 1/4 cup vinaigrette in a large bowl; arrange on a serving platter. Top with grilled steak. Sprinkle with crumbled blue cheese. Serve with remaining 1/4 cup vinaigrette.
LEFTOVER DINNER Steak Wraps: Coarsely chop reserved onions from Grilled Steak Salad with Walnut Dressing. Stir together 4 oz. softened cream cheese and reserved 1/2 cup vinaigrette until smooth. Spread on 1 side of 4 (9-inch) plain wraps. Top each with 10 baby spinach leaves. Divide reserved steak and onions among wraps; sprinkle with 2 oz. crumbled blue cheese. Roll up. Makes 4 wraps.