Melt butter in a Dutch oven over low heat; add onion and cook over medium heat, stirring constantly 5 minutes or until tender.
Whisk in flour until mixture is smooth. Cook 1 minute, whisking constantly. Gradually whisk in half-and-half; cook over medium heat, whisking constantly, until mixture is thickened and bubbly. Stir in salt and next 6 ingredients. Add roe, if desired, and sherry to taste. Garnish, if desired.