Preheat oven to 425°.
Remove and discard giblets and neck from chicken. Rinse chicken with 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 1/2 teaspoon salt and 1/2 teaspoon pepper; rub under loosened skin and over breast and drumsticks. Place oregano, quartered lemon, and celery into body cavity. Lift wing tips up and over back; tuck under chicken. Tie legs together with string. Place chicken, breast side up, on the rack of a broiler pan coated with cooking spray.
Combine remaining 1/2 teaspoon salt, remaining 1/4 teaspoon pepper, butter, onions, and potatoes in a large bowl; toss well to coat. Arrange onion mixture around chicken on rack. Place rack in pan. Bake at 425° for 20 minutes. Reduce oven temperature to 325° (do not remove pan from oven); bake an additional 1 hour and 15 minutes or until onions and potatoes are tender and a thermometer inserted into meaty part of chicken thigh registers 180°. Set chicken, onions, and potatoes aside; cover and keep warm.
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 a small saucepan, stopping before fat layer reaches opening; discard fat. Combine flour and 1/2 cup broth in a small bowl, stirring with a whisk. Add flour mixture and remaining broth to saucepan. Bring to a boil over medium-high heat. Reduce heat to medium; cook 5 minutes or until gravy thickens, stirring frequently with a whisk. Serve gravy with chicken and onion mixture. Garnish with lemon wedges and oregano sprigs, if desired.