In a small bowl, combine the mascarpone, parsley, sliced chives, tarragon, lemon zest, and lemon juice. Mix well and set aside in the refrigerator.

Strain the oysters over a fine-mesh sieve and reserve the oysters and strained liquor separately. Add enough of the clam juice to bring the oyster liquor up to 2 cups.

Heat the butter in a large, heavy pot over medium heat. Add the shallots, leeks, celery, and garlic and cook, stirring frequently, until translucent, about 8 minutes. Sprinkle the flour over the vegetables and cook, stirring constantly, for about 1 minute. Stir in the wine and simmer until thickened and bubbly, about 30 seconds. Add the artichoke hearts, stock, cream, oyster liquor/clam juice mixture, salt, and cayenne and bring to a brisk simmer. Cook for 10 minutes, then remove the soup from the heat and purée in batches in a blender. (Note: use caution when pureeing hot liquids.) Strain the soup through a fine-mesh sieve, pressing to extract as much liquid as possible. Discard the solids and return the soup to a clean pot. Warm the soup gently over low heat until it returns to a simmer.

Add the oysters to the soup and return the soup to a simmer, cooking until the edges of the oysters curl and they are just cooked through, 2 to 3 minutes. Remove the soup from the heat and stir in 1/4 cup of the mascarpone mixture. Adjust the seasoning if necessary. Serve the soup immediately, each bowl garnished with a dollop of the remaining mascarpone mixture, chervil leaves, and chive blossoms.