Preheat oven to 400°.
Trim excess fat from chicken. Remove giblets and neck; reserve for another use. Rinse chicken thoroughly under cold water, and pat dry with paper towels. Sprinkle chicken with salt and pepper.
Loosen skin from breast and drumsticks by inserting fingers and gently pushing between skin and meat. Thinly slice 1 lemon. Place slices and 3 tablespoons sage leaves between skin and meat of chicken. Halve 2 lemons; place in cavity of chicken. Insert meat thermometer into meaty part of thigh, making sure it does not touch bone.
Place remaining sage leaves, onion, and garlic in a large roasting pan coated with cooking spray. Place chicken in pan, breast side up. Combine potato and next 5 ingredients in a bowl; toss gently. Arrange vegetables in pan around chicken.
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°. Remove vegetables from pan; keep 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; keep warm.
Add chicken broth to roasting pan. Cook over high heat, deglazing pan by scraping particles that cling to bottom; cook 5 minutes. Pour broth mixture through a wire-mesh strainer into a 1-cup liquid 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 fre-quently. Cook over medium heat until thick and bubbly, stirring frequently. Stir in Dijon mustard.
Drizzle lemon juice over roasted vegetables. Arrange vegetables around chicken on a serving platter; serve with mustard mixture.