Pick the crab free of shells, a couple times.
Bring a large pot of salted water to a boil and have ready a medium bowl of ice water.
Add the spinach to the boiling water and cook until just tender and still bright green, about 1 minute.
Drain and transfer to the ice bath. When cool, drain again, using your hands to squeeze out excess water.
Chop the spinach, transfer to a bowl, and gently fold in the crab, egg, flour, cayenne, nutmeg, white pepper, and 1 1/2 teaspoons salt.
Using your hands, form the mixture into small, wine-cork-shaped dumplings.
Heat a non-stick skillet over medium-high heat.
Drop in a generous piece of butter, swirl the pan to coat, and allow to begin to foam.
Roll the dumplings in a small amount of flour, shaking off excess, and cook as many dumplings as will comfortably fit in your pan, being careful not to over-crowd them.
Cook until lightly browned, turning as needed, about 6 minutes.
Use a slotted spatula to transfer the dumplings to a platter (or hold them in a low-temp oven to keep them warm).
Finish the dumplings with a drizzle of beurre blanc and serve at once.
MAKE THE BEURRE BLANC
Simmer the wine and salt in a small heavy saucepan until about 2 tablespoons remain.
Slowly whisk in a few cubes of butter at a time, waiting to add the next until each is fully incorporated.
Add the lemon juice and parsley and serve immediately.