Blend flour and hollandaise sauce mix with 2 cups half-and-half in a medium bowl.
Melt butter in a medium, heavy saucepan over medium heat. Slowly add hollandaise mixture, stirring constantly until thickened. Add remaining half-and-half, whipping cream, seafood seasoning, dry mustard, and celery seed. Bring to a simmer and reduce heat to low.
Add crabmeat and sherry, stirring gently until warmed through. Serve immediately.
