Place peppers on an aluminum foil-lined baking sheet.
Broil 5 inches from heat (with electric oven door partially open) about 5 minutes on each side until blistered.
Place peppers in a large heavy-duty zip-top plastic bag; seal and let stand 10 minutes to loosen skins. Peel peppers; remove and discard seeds.
Combine onion, garlic, and 1/4 cup broth in a saucepan; cook, stirring constantly until tender. Stir in sherry; simmer, stirring occasionally, 15 minutes.
Whisk in flour and butter granules; stir in remaining broth and tomatoes. Simmer, stirring occasionally, 15 minutes. Stir in peppers and milk, and cook, stirring constantly, 2 minutes.
Process in batches in a blender until smooth, stopping once to scrape down sides. Stir in sour cream and remaining ingredients. Return to saucepan, and cook over low heat just until thoroughly heated. (Do not boil).