Preheat oven to 400*. Lightly oil roasting pan. Arrange potato rounds on bottom of oiled roasting pan. Season with salt and pepper to taste. Top with onion rounds and rosemary sprig. Cut backbone away from 2 1/2 lb. chicken. Fan out and lay over potatoes and onions, cut side down. Mix garlic, olive oil, and chopped rosemary together in small bowl. Rub over chicken. Add 1 cup of chicken stock to pan. Cover with foil. Bake at 400* for 1 hour. Remove foil. Sprinkle chicken with paprika. Roast uncovered for 25-30 minutes longer. Cut chicken into 4 quarters. Serve with potatoes.