In a saucepan, bring the water, soy sauce, and mirin to a boil.
Add the sweet potato pieces and cook until soft but not mushy, about 3–4 minutes.
Remove with a slotted spoon and allow to cool and drain on paper towels, reserving the cooking liquid. 2.
Add the shiitake mushrooms to the reserved liquid.
Reduce heat and simmer for 5 minutes.
Remove the mushrooms, pat them dry, and allow them to cool before slicing into thin strips. 3.
Place a sheet of nori vertically on a bamboo rolling mat, with the rough side facing up and a short end parallel to you. 4.
Dip your fingertips lightly in cool water and spread about ¾ cup / 150 g (5.3 oz) of the sushi rice evenly over the bottom three-quarters of the nori sheet. 5.
Arrange ¼ of the sweet potato pieces horizontally across the middle of the rice, extending to both edges.
Add ¼ of the mushroom strips and daikon sprouts.
Sprinkle 1 teaspoon (5 ml / 0.2 fl oz) of the toasted almonds over the top. 6. Slide your thumbs underneath the mat and use your fingertips to hold the fillings in place.
Roll the bottom edge of the mat up and just over the fillings, tucking them tightly to start the roll. 7. Lift the edge of the mat and continue rolling forward until the roll is complete and the seam is facing down. Gently shape the roll by pressing with your forefingers on top of the mat while pressing the sides with your thumbs and middle fingers. 8.
Let the roll rest seam-side down on a cutting board for at least 2 minutes.
Repeat to make 3 more rolls. 9. To serve, dip the blade of a very sharp knife in water. Use a swift but gentle sawing motion to cut each roll into 5 even pieces.
Don't overcook the sweet potato! It should be tender enough to bite through easily but still firm enough to hold its shape inside the roll.
Keep a small bowl of water nearby (a 50/50 mix with rice vinegar is even better) to keep your fingers damp while spreading the rice. This prevents it from sticking to your hands.