Trim excess fat from chicken. Remove giblets and neck from chicken; reserve for another use. Rinse chicken thoroughly under cold water, and pat dry with paper towels. Sprinkle chicken with salt and pepper.
Carefully separate skin from body of chicken at neck area, working down to breast and thigh area. Thinly slice 1 lemon. Place lemon slices and 3 tablespoons sage leaves between skin and meat of chicken. Cut remaining 2 lemons in half; place in cavity of chicken. Truss chicken. Insert meat thermometer into meaty part of thigh, making sure it does not touch bone.
Place remaining tablespoon sage leaves, sliced onion, and garlic in a shallow roasting pan coated with cooking spray. Place chicken, breast side up, on top.
Combine potatoes, sweet potato, carrot, and onion wedges in a large bowl. Combine brown sugar and lemon rind; sprinkle over vegetables, and toss gently. Arrange vegetables in a shallow roasting pan coated with cooking spray.
Bake chicken and vegetables, uncovered, at 400° for 15 minutes. Reduce heat to 350°; bake, uncovered, 1 hour and 30 minutes or until vegetables are tender and meat thermometer registers 180°. Keep vegetables warm.
Transfer chicken to a serving platter; remove and discard skin, lemon slices, and sage. Remove and discard lemon halves from cavity. Set chicken aside, and keep warm.
Add chicken broth to roasting pan. Cook over high heat, deglazing pan by scraping particles that cling to bottom; cook 5 additional minutes. Pour broth through a wire-mesh strainer into a 1-cupliquid measuring cup, discarding onion, sage, and garlic. Skim fat from broth; add water to make 1 cup.
Combine flour and 1/4 cup broth mixture in a small saucepan, stirring until smooth. Gradually add remaining broth mixture, stirring frequently. Cook over medium heat, stirring frequently, until thickened and bubbly. Stir in Dijon mustard.
Drizzle lemon juice over roasted vegetables. Arrange vegetables around chicken on a serving platter; serve with