Boil, peel and mash potatoes. Chop eggs; add to potatoes. Combine mayonnaise, parsley flakes, celery salt, green onion, salt and pepper. Mix with egg and potato mixture. Chill thoroughly.
FOR FILLIING: Soften Neufchatel cheese and mix with crushed egg yolk, mayonnaise, paprika and salt. Line a 12 x 9-inch pan with foil. Pat chilled potato mixture onto foil about 1/4-inch thick to about 1-inch of foil edge. Spread filling mixture on top. Lift foil and gently roll up in jellyroll fashion. Score roll with fork. Chill thoroughly. Garnish with parsley. Cut slices from chilled roll.