In a small bowl combine the mayonnaise, mustard, soy sauce, lemon juice, and horseradish and stir to combine well. Set aside.
Hold one sheet of seaweed in your hand and with the other hand (fingers should be damp) scoop a small handful of rice and press onto seaweed, leaving 2 of the diagonal corners free of rice (these will be the top and bottom of the handroll). Lay 2 pieces of soft-shell crab, several slices of avocado, and some of the cucumber on top of the rice, and sprinkle with scallions and baby cress. Carefully begin rolling the roll, ice cream cone-fashion. If the roll does not stay closed on its own, you can either wet one edge or place a few stray pieces of rice to act as a glue . Serve immediately, with some of the sauce spooned over the top. Repeat with remaining ingredients.
Tempura Soft-shell Crabs:
Preheat the deep-fat fryer.
To clean the crabs, remove the tail, gills, and eyes. Season with salt and pepper.
Whisk together the essence, egg, flour, cornstarch, soda water, salt, and pepper into a smooth batter. Dip the soft-shell into the tempura batter, shaking off any excess. Remove the basket from the fryer. First, carefully holding the top of the crab, drag the legs through the oil first, for 1 minute. This will allow the individual legs to fry separately instead together. Then lay the soft-shell, top-side-down, into the oil. Fry for 2 to 3 minutes or until golden, flip the soft-shell over and continue frying for additional 2 to 3 minutes. Remove from the fryer and allow to drain on a paper-lined plate.
Mitsuko's Perfect Sushi Rice:
In a large bowl, add rice and cover with lots of water. Stir rice in water with your hands, then pour off most of the water, and agitate 10 to 15 times by quickly moving your hand back and forth, swishing rice against side of bowl. Rinse the rice several more times, until the water that drains off of the rice is almost clear. Transfer the rice to a colander and let drain for 1 hour, undisturbed. Transfer the rice to a rice cooker, add the water, cover, and cook as directed by manufacturer?s directions.
While the rice is cooking, bring rice vinegar to a boil in a small saucepan, and then remove from the heat. Add sugar and salt and stir until dissolved. Allow to cool to room temperature.
When rice is done, allow cooked rice to remain in rice cooker for 10 minutes. Transfer rice to a large shallow mixing bowl (traditionally a wooden tub, called a hangiri), so that rice forms a mound in the center of the bowl. Using a diagonal slicing motion, gently cut into rice with a wooden paddle (called a kijakushi), and pour cooled vinegar mixture over top of rice. ?Cut? rice several times to evenly distribute vinegar mixture, then allow to cool. Gently turn rice over from time to time with paddle so that rice cools evenly. You might want to consider using a fan if time is of the essence. When rice has cooled to body temperature, it is ready to use for sushi rolls. Rice can be kept at the proper temperature by placing it in an insulated container such as a small ice chest, covered with a damp kitchen towel.
Yield: enough rice for at least 7 large rolls (futomaki)