Whisk mustard, olive oil, parsley, and thyme in small bowl to blend. Stir in garlic. Spread mustard mixture generously inside cavities and over outside of chickens. Place chickens in glass baking dish; cover and refrigerate at least 4 hours or overnight.
Let chickens stand at room temperature 30 minutes. Sprinkle chickens generously inside and out with salt and pepper; let stand 30 minutes longer.
Preheat oven to 400°F. Place chickens on rack set in large roasting pan. Tie legs together to hold shape. Spread any accumulated mustard marinade from dish over chickens. Place rosemary and sage sprigs atop chickens. Roast chickens until juices run clear when fork is inserted into thickest part of thigh, basting occasionally with pan juices, about 1 hour 10 minutes.
Divide Warm Frisée, Fingerling, and Bacon Salad among 6 plates. Carve chickens and arrange atop salad. Spoon some of pan juices over and serve.