Preheat oven to 325°.
To prepare chicken, remove and discard giblets and neck from chicken. Rinse chicken under cold water; pat dry. Trim excess fat. Starting at neck cavity, loosen skin from breast and drumsticks by inserting fingers, gently pushing between skin and meat. Combine thyme, orange juice, 1/4 teaspoon salt, 1/4 teaspoon pepper, and garlic; rub over breast and drumsticks under loosened skin. Place orange halves in body cavity. Lift wing tips up and over back; tuck under chicken.
Combine 6 garlic cloves, 2 thyme sprigs, and 1 cup of broth in a shallow roasting pan. Place chicken on a rack coated with cooking spray; place rack in pan. Insert meat thermometer into the meaty part of thigh, making sure not to touch bone.
Bake at 325° for 2 hours or until thermometer registers 180°. Let stand 10 minutes. Discard skin. Place chicken on a platter, reserving drippings in pan; cover chicken and keep warm. Add wine to drippings in pan, scraping pan to loosen browned bits; discard thyme sprigs.
Place flour in a small saucepan; gradually add remaining broth, stirring with a whisk until blended. Set aside. Place a zip-top plastic bag inside a 2-cup glass measure. Pour pan 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 into flour mixture in saucepan, stopping before fat layer reaches opening; discard fat. Bring mixture to a boil, and cook 1 minute or until thick, stirring constantly with a whisk. Keep warm.
To prepare mushrooms, melt butter in a large nonstick skillet over medium-high heat. Add 1/2 teaspoon salt, 1/4 teaspoon pepper, and mushrooms; cook 5 minutes or until the mushrooms release moisture and darken, stirring frequently. Remove from heat; stir in truffle oil.
To prepare polenta, combine milk, water, 1/2 teaspoon salt, and 1/4 teaspoon pepper in a medium saucepan over medium-high heat; bring to a boil. Gradually add polenta, stirring constantly with a whisk; reduce heat, and cook 2 minutes or until thick, stirring constantly. Remove from heat, and stir in cheese. Serve with chicken, gravy, and mushrooms.