1. To prepare chicken, heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Add lemon to pan; sauté 5 minutes or until lightly browned, turning occasionally. Remove from pan; cool.
2. Preheat oven to 400°.
3. Remove and discard giblets from chicken; trim excess fat. Starting at neck cavity, loosen skin from breast and drumsticks by inserting fingers, gently pushing between skin and meat. Combine 1/4 teaspoon salt and 1/4 teaspoon pepper; rub salt mixture under loosened skin. Insert lemon slices in a single layer under loosened skin. Place rosemary inside body cavity. Lift wing tips up and over back; tuck under chicken. Place chicken on a roasting pan coated with cooking spray. Lightly coat chicken with cooking spray. Remove white papery skin from garlic head (do not peel or separate cloves). Place garlic head in pan.
4. Bake at 400° for 15 minutes. Reduce oven temperature to 350° (do not remove chicken from oven); bake an additional 1 hour or until a thermometer inserted in the meaty part of thigh registers 165°. Remove chicken from pan; let stand 10 minutes. Remove skin; discard. Loosely cover chicken; keep warm.
5. Remove garlic from pan; set aside. Place a zip-top plastic bag inside a 2-cup glass measure. Pour drippings into bag; let stand 10 minutes (fat will rise to the top). Seal bag; carefully snip off 1 bottom corner of bag. Drain drippings back into pan, stopping before fat layer reaches opening; discard fat.
6. Separate garlic cloves; squeeze to extract pulp. Place pan over medium-high heat. Stir in garlic pulp, wine, and broth; bring to a boil, scraping pan to loosen browned bits. Cook 10 minutes or until sauce is slightly thick. Remove from heat; stir in butter.
7. To prepare potatoes, place potatoes in a saucepan; cover with water. Bring to a boil. Reduce heat, and simmer 20 minutes or until very tender; drain. Return potatoes to pan. Mash with a potato masher to desired consistency. Add milk and remaining ingredients; stir until combined. Serve chicken with gravy and potatoes.