How to Make It
Melt the butter in a medium nonreactive saucepan over moderate heat. Add the flour and cook, whisking for 30 seconds, Add the half-and-half and cook, whisking constantly until very thick, about 1 minute. Remove from the heat and let cool. Fold in the crabmeat, scallion, tarragon and mustard; season with salt and pepper. Divide the crabmeat filling into equal portions.
On a work surface, lightly brush 3 wonton wrappers with water. Place 1 portion of the crab filling in the center of each. Cover with a second wonton wrapper, press out any air pockets and seal. Using a 2 1/2-inch round or fluted biscuit cutter, trim the excess dough. Transfer the ravioli in a single layer to a baking sheet lined with plastic wrap. Repeat with the remaining wrappers and filling.
Melt the butter in a large nonreactive skillet. Add the shallot and cook over low heat, stirring, until softened but not browned, about 4 minutes. Add the cream and clam broth and bring to a boil over high heat. Off the heat, whisk in the mustard and parsley and season with salt and white pepper. Cover and set aside.
Cook the ravioli in a large pot of boiling salted water, stirring occasionally, until they rise to the surface, about 3 minutes. Using a slotted spoon, transfer the ravioli to a colander to drain. Add the drained ravioli to the shallot cream and toss gently over low heat. Serve immediately.