Melt butter in saucepan.  Stir in flour.  Add seafood stock and seasonings and simmer 2 minutes.  Add whole milk and evaporated milk; cook slowly stirring constantly, until thickened.  Do not let mixture boil.  Add crabmeat after mixture thickens.  Cook on low about 10 minutes.  Remove from heat; garnish with green onion tops.  Serve immediately.