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1
Rinse scallops and sole and pat dry, then puree in a food processor until very smooth.
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2
Force puree through a fine-mesh sieve into a metal bowl, scraping bottom of sieve as needed.
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3
Set metal bowl in a larger bowl of ice and cold water, then add egg white to puree and stir vigorously with a wooden spoon or rubber spatula until well combined.
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4
Add cream 2 tablespoons at a time, stirring after each addition until incorporated.
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5
(Mousseline should be the consistency of soft mashed potatoes; if it becomes runny or separates, stop adding cream and chill mixture, covered still in ice bath until firmer, about 30 minutes.
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6
Then continue adding cream.)
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7
Cover bowl and chill mixture 8 hours.
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8
Put oven rack in middle position and preheat oven to 350F.
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9
Cut out 10 rounds of parchment paper to line bottoms of molds and 10 rounds to line tops of molds, using 1 timbale mold as a guide.
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10
Brush molds with some melted butter and line bottom of each with a round of parchment.
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11
Chill molds 5 minutes (to set butter), then brush paper and sides of molds again with more melted butter.
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12
Stir salt, pepper, nutmeg, chives, and dill into mousseline, then divide among molds (they will be about two-thirds full).
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13
Rap molds on counter once or twice to settle mixture, then put a buttered parchment round, buttered side down, on surface of each mousseline.
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14
Put molds in an 8-inch square or round baking pan (2 inches deep) and bake in a hot water bath in oven until mousselines are just set and springy to the touch, 20 to 25 minutes.
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15
Cool mousselines in molds on a rack until warm, about 10 minutes, then invert onto a large plate and pat dry with paper towels before transferring to soup bowls.
