In a heavy skillet, heat 4 tablespoons of butter. Add the onion, green pepper, and mushrooms. Sauté until onions are softened. Add the green onion and parsley and continue cooking for 2 minutes longer. Set aside.
In a medium saucepan, heat the remaining 2 tablespoons of butter over medium heat. Stir in the flour until smooth; gradually stir in the milk. Add salt, pepper, nutmeg, and cayenne. Cook, stirring, until thickened. Add the sautéed vegetables and half-and-half or light cream, and cook, stirring, until hot and bubbly. Stir in the cooked crabmeat and simmer for 5 minutes. Stir in the sherry just before serving.