In a braiser or sauté pan large enough to hold all the chicken pieces in one layer, gently sauté the onion in abundant olive oil until it has softened and turned translucent. It's a good idea to add a pinch of salt and a few drops of water while the onion cooks to speed the cooking and avoid browning.

Turn up the heat to medium and add the chicken pieces. Sauté the chicken, turning them from time to time, until they are lightly browned on all sides. Take care not to burn the onion. Season the chicken pieces generously.

Add the white wine, scrapping up any bits of chicken that may have stuck to the bottom of the pan, and let the wine evaporate.

Then add the broth, along with the rosemary and sage, turn down the flame to low, and cover. Let the chicken braise for a good 30-45 minutes, until the chicken is tender and completely cooked through. (It's a good idea to remove the chicken breasts about midway to avoid their overcooking, then put them back just before proceeding to the next step.) There should be ample cooking liquid throughout; add more broth if needed.

Uncover the pan, remove the rosemary and sage, and add the egg yolks and lemon juice, which you will have beaten together in small bowl, stirring until the cooking liquid has thickened into a light, creamy sauce.

Serve right away, with some good quality crusty bread.