Preheat oven to 400 F. In small roasting pan or baking dish, combine potatoes, fennel, lemon juice, olive oil, salt and pepper. Spread mixture evenly over bottom of pan. Set aside.
Place slice of bread in bowl of food processor. Process until bread becomes crumbs the size of peas. In small bowl, combine mayonnaise, mustard, curry powder and tarragon.
Season chicken with salt and pepper. Place 1 teaspoon of mayonnaise mixture under skin of each chicken breast. Spread 2 rounded teaspoons of the mayonnaise mixture on top of skin; spread evenly to coat. Sprinkle each breast with 1/4 of the bread crumbs, pressing to adhere to the chicken.
Place chicken on top of potatoes and fennel. Place in oven and roast for about 45 minutes, or until meat thermometer registers 170 F.
Serve chicken over fennel and potatoes.