Step 1

Combine the scallops and the egg in a food processor; sprinkle the salt and white pepper over them. Turn on the processor and add the cream through the feed tube. Process until the mixture is smooth; about 10 seconds total. Don’t overprocess or the mousse will turn out rubbery. With a stiff rubber spatula or scraper, force the mixture through a medium-fine mesh sieve into a bowl. Check the seasoning by steaming a dab of mousseline for 1 to 2 minutes or until cooked through. Taste and adjust seasonings if needed. Cover and chill the mousse until it’s time to assemble the timbales.

Step 2

Butter the bottom and sides of eight 4-oz. molds. If the sole fillets are more than 1/2 inch thick, split each one lengthwise to make 1/4-inch-thick fillets.

Step 3

Wrap a ramekin tightly in plastic. Using the bottom, gently press each fillet to a uniform thickness of 1/8 inch. Start near the center and gently push outward toward the edges, taking care not to tear the fish. Using a sharp 2-inch cookie cutter (or one that’s equal in diameter to the inside of the mold you’re using), mark circles in the fish.

Step 4

With kitchen shears, cut out the circles. Make as many circles as you can and save all the scraps.

Step 5

Take the eight neatest circles and place one in each buttered timbale mold, with the side the skin was on facing up (the bottom of the ramekin becomes the top of the finished, unmolded timbale). Lightly season each with salt and pepper. Using a pastry bag with a plain tip (or no tip), pipe or spoon a 3/8-inch-thick layer of scallop mousseline into each mold.

Step 6

Using the rest of the sole circles if you have any, as well as the sole scraps, add another layer of sole to each mold, cutting small pieces to fit any gaps so that the layers are even. Gently press on the sole to even out the mousseline underneath and to force out any air pockets. Again, season lightly with salt and pepper. Repeat with another layer of mousseline. Season lightly. Finish with a final layer of sole. Seal each mold with plastic wrap; refrigerate.

Step 7

Put the shallot, wine, vinegar, peppercorns, and thyme in a nonreactive saucepan. Simmer, reducing, until 1 Tbs. liquid remains. Remove from the heat and reserve. (You can make the reduction up to an hour ahead.)

Step 8

Melt the butter in a small saucepan over low heat. Add the thoroughly rinsed, sliced leeks (there’s no need to dry them). Cover the pan and cook very gently, adding water if the pan gets too dry (the leeks shouldn’t brown). Cook until very soft and tender, 20 to 25 minutes. Set aside and keep warm until it’s time to serve the timbales.

Step 9

Heat the oven to 325°F. Put the wrapped timbales in an ovenproof pan large enough to hold all the molds. Fill the pan with enough hot water to come halfway up the sides of the molds.

Step 10

Bake until the timbale feels firm but is still jiggly, 20 to 23 minutes (it will have shrunken slightly from the sides of the mold and will be quite tender even when fully cooked). Insert a paring knife into the underside of a timbale; the knife tip should come out warm when you touch it to your lip.

Step 11

Heat the reduction over low heat. When the pan is warm and the liquid is just about gone, whisk in the butter a few pieces at a time, whisking constantly to get a smooth emulsion. Strain the sauce through a fine mesh sieve into a clean saucepan, pressing on the solids and then discarding them. Stir in the chives, taste the sauce, and adjust as needed with salt, white pepper, and a few drops of lemon juice. Set the beurre blanc over a barely simmering water bath to keep it warm.

Step 12

Divide the braised leeks evenly among eight warmed plates. Unwrap the timbales and turn them out onto a flat plate lined with a kitchen towel. Gently blot each timbale dry and set each one on the braised leeks. Ladle a generous 2 to 3 Tbs. beurre blanc onto each timbale. Garnish each with 1/2 to 1 tsp. caviar and a tiny sprig of chervil.