Melt 2 tablespoons of the butter in a sauce pan over medium heat. Add the green onions, celery and garlic and saute until slightly softened. Remove form the heat.
In a seperate pan over medium heat, melt remaining butter and whisk in the flour. Whisk in the half & half, and bring just to a simmer, being careful no to boil it. Reduce heat to low and add the seasonings (parsley through black pepper). Add the vegetable mixture and the oysters and scallops. Simmer for 15 minutes, stirring frequently.
Add the Worcestershire and sherry and contimue to simmer until the stew thickens.