Check oysters for shells, drain and reserve liquid. Cut oysters in half if large. Make roux with butter and flour. Stir together in 3 quart saucepan over medium heat until color of peanut butter. Add oysters to roux. Stir over medium heat until edges curl. Add garlic, onion tops and artichokes.
Slowly add 3/4 cup reserved oyster liquid and simmer 10 minutes. Add sherry, then bread crumbs until consistency of a chowder (not too dry). Add salt, pepper, thyme and oregano. Place mixture into individual ramekins with a slice of lemon on top. Bake in preheated 400F oven 10-15 minutes or until bubbly.