Preheat oven to 425°.
Remove and discard giblets and neck 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 1/2 teaspoon salt and 1/2 teaspoon black pepper; rub under loosened skin and over breast and drumsticks. Place oregano sprigs, quartered lemon, and celery pieces 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 1/2 teaspoon salt, remaining 1/4 teaspoon pepper, melted butter, onions, and potatoes in a large bowl, and 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 10 minutes or until onions and potatoes are tender and a thermometer inserted into meaty part of chicken thigh registers 165°. 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 remaining 1/4 teaspoon salt, flour, and 1/2 cup chicken broth in a small bowl, stirring with a whisk. Add flour mixture and remaining chicken 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. Carve chicken; serve with gravy and onion mixture. Garnish with lemon wedges and oregano sprigs, if desired.