1. Remove neck and giblets from chicken. Transfer neck to medium roasting pan; discard giblets. Pat dry with paper towels. Sprinkle cavity with half of salt and pepper. Place lemon inside cavity. Arrange onion slices in center of roasting pan.
2. Place the chicken on onions. Fold wing tips under; loosely tie legs with kitchen twine.
3. Preheat oven to 425°F. Sprinkle chicken with remaining salt and pepper. Roast 1 hour 15 minutes, rotating pan halfway. Raise temperature to 450°F. Continue roasting until meat thermometer inserted into the thickest part of the breast registers 165°F (20-30 minutes).
4. Transfer chicken and onion to platter. Discard chicken skin.
5. Place pan over high heat. Bring liquid in pan to a boil. Add 3/4 cup water. Return to a boil, scraping browned bits. Cook until reduced by one-third (5 minutes). Pour into cup or fat separator. Let sit until fat rises to surface (5 minutes). Discard fat. Serve sauce with chicken.