Arrange tomatoes, cut sides up, in 1 layer in a large shallow baking pan (with sides) and add garlic to pan. Drizzle tomatoes with oil and sprinkle with salt, pepper, celery flakes, and red pepper flakes. Roast seasoned tomatoes for 1 hour, then cool completely in pan. Peel garlic (the skins should easily slip right off).
In a 6 to 8 quart heavy sauce pot, cook onion, oregano, and sugar in butter over low heat, stirring often, until onion is softened and slightly caramelized, about 5 minutes. Add tomatoes, garlic, and stock and simmer, covered, 20 minutes.
Purée soup in batches in a blender (use caution when blending hot liquids), then force through a sieve into cleaned pot, discarding solids. Stir in cream and salt and pepper to taste and simmer 2 minutes.
Divide soup among 8 bowls and float 1 wafer in center of each.
*Soup can be made 1 day ahead and cooled, uncovered, then chilled, covered. Reheat just before serving.