Step 1Separate the Belgian endive into individual leaves, stack them into small bunches and slice them lengthwise into very thin slivers. Stir them together in a medium bowl with the crab meat and almonds.
Step 2In a small bowl, beat together the mayonnaise and mustard. Add enough to the crab meat and endive to bind them together lightly, about 3 tablespoons.
Step 3Cook the rice vermicelli in a large pot of rapidly boiling, salted water until tender, 2 to 3 minutes. Drain and place in a bowl filled with water to keep them pliable. When you're ready to work, drain them in a strainer, shaking off as much water as possible.
Step 4Fill a large dinner plate or pie plate with water. Slip in one of the rice paper wrappers and press down lightly to keep it submerged. Let it soak until the wrapper just becomes pliable (it may still feel a little stiff), about 30 seconds.
Step 5Here's where it gets a little tricky -- you need to keep the filling packed as tightly as possible to have a nicely formed roll, but you also need to work carefully to avoid tearing the wrapper or having it stick together. Remove the wrapper from the water and spread it on a work surface. Gently blot it dry with a paper towel. Arrange a row of half a dozen watercress leaves down the center of the wrapper about one-third of the length of the wrapper (this will show through as a decoration when the rolls are completed).
Step 6Using a slotted spoon and straining out any extra liquid, place 2 to 3 tablespoons of the crab-endive mixture on top. Top with one-fourth to one-third cup of the rice noodles.
Step 7Pull the far edge over the filling and roll once, pushing the mixture against the secure edge to seal tightly (here the rice paper's stickiness comes in handy). Fold the sides over like a burrito, compacting the filling in the center. Finish rolling, keeping the filling as tight and compact as possible. Transfer the finished roll to a plate and repeat with the remaining filling. (The spring rolls can be prepared to this point up to a day in advance and refrigerated, tightly covered with plastic wrap. Bring to room temperature before serving.)
Step 8To serve, cut the rolls in half on the diagonal.