Put chopped onion and garlic on a chopping board and sprinkle with a pinch of salt. With back of a heavy spoon, press into a semi-paste. Combine all ingredients except last three in a bowl and toss well.
Slice lettuce crosswise into 1-inch strips. Whisk balsamic vinegar and extra-virgin olive oil together and toss with lettuce to coat well. Arrange on 4 plates and top with chicken mixture.
Makes 4 servings.