Directions

1. Place chicken on large heat-proof plate. Sprinkle with tarragon and garlic, and season with salt. Arrange lemon slices over chicken, and top each breast with a lettuce leaf.
2. Place a round cake rack in a 12-inch straight-sided skillet. Add enough boiling water to come 1 inch up the sides of the skillet. Place plate with chicken on the rack. Cover, and simmer until chicken is cooked through, 25 to 30 minutes. (Check the pan after 15 minutes to make sure there is still water in the bottom.)
3. Carefully remove chicken from plate. Discard lettuce leaves. Spoon about 1/4 cup of the accumulated juices on the plate into a small bowl. Whisk in mustard to make a sauce. Spoon sauce over chicken, and serve.