In A Large Bowl, Combine The Ground Beef, 2 Tablespoons Dressing Mix, And 1/4 Cup Sour Cream; Mix Well (The Mixture Will Be Soft). Form The Mixture Into 4 Burgers. Grill The Burgers, Turning Once, Until Done (10 To 12 Minutes); Set Aside. In A Small Bowl, Combine The Remaining Dressing Mix And Remaining Sour Cream; Mix Well. Place Each Grilled Burger On A Bun. Top Each Burger With A Lettuce Leaf And 1/4 Of The Diced Tomatoes And Onions. Place A Dollop Of Dressing On Top Of Each Burger Before Serving.