Remove and discard giblets and neck from chicken. Place 2 crushed garlic cloves, rosemary sprigs, and lemon in body cavity. Combine chopped rosemary, 3/4 teaspoon salt, and olive oil. Starting at neck cavity, loosen skin from breast and drumsticks by inserting fingers, gently pushing between skin and meat. Rub oil mixture under skin. Lift wing tips up and over back; tuck under chicken. Tie legs together with twine. Place chicken, breast side up, in a roasting pan.
Bake at 425° for 25 minutes. Arrange shallots and pears around chicken. Add broth to pan; baste chicken. Bake 30 minutes. Stir pears and shallots; baste chicken. Add walnuts to pan. Bake an additional 10 minutes or until a thermometer inserted in the meaty part of thigh registers 165°. Let stand 20 minutes.
Place a zip-top plastic bag inside a 2-cup glass measure. Pour drippings from pan into bag; add 1/2 cup water. Let stand 2 minutes (fat will rise to the top). Seal bag; carefully snip off 1 bottom corner of bag. Drain drippings into measuring cup, stopping before fat layer reaches opening; discard remaining fat. Return drippings to pan, and cook over medium heat, scraping pan to loosen browned bits. Add remaining 1/4 teaspoon salt, vinegar, honey, juice, and 1 minced garlic clove; cook until reduced to 1/2 cup (about 4 minutes). Remove from heat; stir in walnut oil.
Remove skin from chicken; discard skin. Carve chicken; arrange chicken and pear mixture on a platter. Serve with sauce.