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1
Preheat broiler.
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2
Quarter peppers lengthwise, discarding stems, seeds, and ribs.
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3
Put skin sides up on rack of a broiler pan, and broil about 2 inches from heat until skins are blistered and charred, 8 to 12 minutes.
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4
Transfer peppers to a bowl and let stand, covered with plastic wrap, until cool enough to handle.
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5
Peel peppers and thinly slice.
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6
Cook onions in butter in a 5- to 6-quart heavy pot over moderately low heat, covered, stirring occasionally, until soft.
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7
Stir in peppers, stock, and herbes de Provence and simmer, uncovered, 20 minutes.
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8
Puree soup in a blender in batches (use caution when blending hot liquids).
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9
For a smoother texture, force puree through a food mill into a large bowl.
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10
Return soup to pot and season with salt, tarragon, and cayenne.
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11
Reheat soup.
