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1
Cook cucumbers in a 4- to 5-quart pot of boiling salted water , uncovered, just until water returns to a boil, about 1 minute, then immediately drain in a colander and transfer to a bowl of ice and cold water to stop cooking.
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2
Drain cucumbers well, then puree in 2 batches in a blender until as smooth as possible, about 1 minute, transferring to a bowl.
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3
Peel potato and cut into 1/2-inch cubes.
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4
Cook onion, carrot, celery, garlic, bay leaf, and thyme in butter in a 2-quart heavy saucepan over moderate heat, stirring occasionally, until vegetables are softened, about 3 minutes.
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5
Add potato, 4 cups water, pepper, and 1/2 teaspoon salt and simmer, partially covered, until potato is very tender, about 15 minutes.
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6
Discard bay leaf and thyme.
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7
Puree mixture in batches in blender until very smooth, about 1 minute (use caution when blending hot liquids), transferring to a large bowl.
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8
Chill, uncovered, until completely cool, about 30 minutes.
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9
Stir cucumber puree into potato puree and force soup through a fine-mesh sieve into a large bowl, pressing hard on and then discarding solids.
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10
Stir remaining teaspoon salt into soup, then reheat in a bowl in a microwave or in a saucepan on stovetop over low heat (microwaving is best because it preserves the soup's bright green color).
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11
Put 1 mousseline in each of 10 shallow soup bowls and pour warm soup around each.
