Combine vinegar, oil, rosemary, thyme, salt, and pepper in jar with tight-fitting lid. Cover and shake well.
Brush onion on both sides with some vinegar mixture. Grill onion and frozen burgers over medium heat for 4 to 6 minutes, turning once. Remove from grill.
Arrange lettuce on individual plates. Cut burgers into strips. Separate onion into rings. Place burgers and onion on top of lettuce. Sprinkle with blue cheese and walnuts. Shake vinegar mixture. Drizzle over salads.
Helpful Hints Toasting enhances nuts' natural flavors, making them irresistible. As a general rule, to toast most varieties of nuts, spread them in a single layer in a shallow baking pan. Bake at 350°F for 5 to 10 minutes or until light golden brown. Be sure to watch carefully and stir them once or twice so they don't burn.