Greek Gyro

2 lb Lean, boneless lamb*
-- cut in 1" cubes
1/3 c Olive oil
1/2 c Fresh lemon juice
2 Garlic cloves
-- peeled, cut in quarters
1 ts Chopped fresh thyme
1 ts Chopped fresh oregano or
1 ts Chopped fresh marjoram
Fresh ground pepper to taste
2 Spanish onions; cut in
-thinly sliced rings
24 sm Romaine lettuce leaves
-- washed and dried
6 lg Pita bread rounds
3 md Ripe tomatoes