In a large bowl, add in chicken and pesto, coat chicken with pesto. Allow to sit for ten minutes to an hour in the fridge.
Add marinated chicken to the grill at medium high heat, cook each side for 12-15 minutes, or until cooked thoroughly. Chop cooked and slightly cooled chicken, into bite sized chunks.
In a quart mason jar, in this order, layer in balsamic vinaigrette, chicken, tomatoes, mozzarella cheese, onions (optional), and spring lettuce. Be sure to gently tap the mason jar between layers, so everything packs in well.