Mix all the ingredients for the sauce with an immersion blender or in a food processor.
Reserve the tonnato sauce in order to let the flavors develop.
Put the chicken breasts in a pot with just enough lightly salted water to cover it.
(If you’re using precooked chicken then skip this step.) Bring the water to a boil and discard the foam appearing on the surface.
Let simmer on medium heat for about 15 minutes or until the chicken is fully cooked through.
If you’re using a meat thermometer, it should say 165°F (75°C) when done.
Pour the sauce over the chicken and serve with extra capers and a wedge of fresh lemon.