Step 1Heat the oven to 350 degrees.
Step 2Mix the onion and rosemary in a small bowl. Spread the mixture between the skin and flesh of the chickens. Place the chickens on vertical roasting stands, wings down, in a roasting pan with the water and bake 50 minutes, until the juices run clear when the thighs are pierced with a fork. Be sure that the water level remains constant in the pan. Add more water if water level is too low.
Step 3Remove the chickens from the oven. When they're cool enough to handle, split them into halves, and cut the bone away from the meat, starting at the breast bone and cutting each breast away from the ribs. Remove the thigh bone. Leave only the leg bone intact. Cut each half into 3 horizontal sections, making 6 sections in all. Set aside.
Step 4Mix the lettuce and frisee in a large bowl, then distribute onto individual plates.
Step 5Cook the mushrooms in the butter over medium-high heat until soft, 2 minutes. Place the tomatoes and mushrooms on top of the lettuce. Top each plate with the chicken. Serve with Dressing.
Step 1Combine the mustard, whipping cream and vinegar in a medium mixing bowl. Using a metal whisk, whip the cream mixture until it's smooth and slightly thickened, 1 to 2 minutes. Serve on the side with the salad.