Combine oil, rosemary, garlic and onion in a large bowl. Add the chickens and turn to coat, cover and refrigerate at least 2 hours or overnight. Preheat oven to 400 degrees F. Remove chicken from marinade, truss, and season with salt and pepper to taste. Heat roasting pan over high heat and sear bird. Place in the oven and roast for 10 minutes. Reduce the heat to 350 degrees F and continue roasting, basting with the pan juices until the chicken is cooked through and golden brown, 30 to 35 minutes more.
Preheat oven to 350 degrees F. Heat a medium saute pan over high heat until almost smoking. Rub the cut side of the lemon with olive oil and sear in the hot pan cut side down. Turn the lemons over and place the pan in the oven and cook until almost soft. Reserve and serve as a garnish.
Place lemon juice in a nonreactive pan over high heat and reduce to 1/4 cup. Heat 2 tablespoons of olive oil in a medium saucepan over medium heat. Add the carrots, celery and onions and cook until soft. Raise the heat to high and add the wine and the port and cook until dry. Add the chicken stock and cook to a sauce consistency. Strain through a fine strainer into a clean saucepan. Add the reduced lemon juice and cook for 2 to 3 minutes. Add the capers and butter and season with salt and pepper to taste.
Coarsely grate the potatoes. Dry thoroughly by rubbing them between two clean kitchen towels. Place the grated potatoes in a large bowl, mix in the herbs and season with salt and pepper to taste. Heat the oil in a 10-inch nonstick skillet over medium high heat until almost smoking. Add the potatoes in a flat even layer. Shake the pan frequently to prevent the galette from sticking. After 3 to 4 minutes, invert the potatoes onto a plate and carefully slide them back into the pan, reduce the heat to low and continue cooking for another 20 to 30 minutes. Cut into wedges.