Cut the leeks in half lengthwise and hold them under running water while flipping through the leaves to rinse out any sand. Cut the leaves 2 or 3 at a time into fine julienne. If the leaves are more than 4 or 5 inches long, fold them under to make them easier to julienne.
Julienne the carrots and turnip into 3- inch lengths. The easiest way to do this is first to slice them with a benriner cutter (a Japanese slicer) and then with a chef's knife. Cut the celery stalk into 4-inch sections and julienne it as finely as possible. Combine the julienned vegetables, bouquet garni, and just enough water to cover- about a quart- in a 4-quart pot. Simmer gently, partially covered, for 15 minutes. Add the wine and simmer for 10 minutes more. Strain the vegetable broth, reserving the cooked vegetables and broth. About 10 minutes before you're ready to serve, put the cooked vegetables with 1/4 cup of the broth in a covered saucepan over low heat.
Bring the remaining vegetable broth to a simmer in a saute pan just large enough to hold the scallop in a single layer. (If the pan is too large, there won't be enough liquid to cover.) Gently slide the scallops into the simmering broth. If there's not enough liquid to cover the scallops. Poach the scallops for 8 minutes. Spread 3/4 quarters of the julienned vegetables in shells. Carefully arrange the scallop on the vegetables.
Add the chopped parsley to the simmering poaching liquid, season the liquid with salt and pepper, swirl in 1/4 cup tarragon butter and ladle enough over the fish to come about a third of the way up the sides of the fish. Arrange the rest of the julienne over the fish.