Rub chicken with olive oil and salt and pepper. In a large baking dish, place chicken and leek and layer sliced lemons on top.
Place baking dish in the oven and bake for 15 to 25 minutes, until chicken is no longer pink in the center. Remove lemons, and allow chicken to cool. When chicken has cooled, chop.
In a large bowl, layer lettuce, avocado and chicken. In a small bowl, whisk the remaining 4 ingredients (lemon juice through chives) and pour over the salad. Toss and serve.