1) For burgers, crumble ground beef in medium mixing bowl. Finely chop onion. Add onion, Worcestershire sauce, salt and black pepper to ground beef; mix gently but thoroughly. Form mixture into 8 round patties. 1/2 inch thick. Refrigerate covered until ready grill.
2) For Mexican sandwich spread, combine mayonaise, sour cream and seasoning mix in small bowl; mix until well blended. Cover and refrigerate.
3) For Mexican toppers, slice tomatoes. Slice avocado. place toppers in individual serving bowls; cover and refrigerate.
4) Grill patties, uncovered, 11-13 minutes or until centers are no longer pink, turning once. About 1 minute before burgers are done, place cheese slices on top.
5) Meanwhile, place roll halves, cut side down, on grill. Grill 1-2 minutes or until rolls are lightly toasted. Place burgers in rolls. Serve with spread and toppers.