In a medium bowl, gently toss the grated raw beet with your prepared sushi rice until the mixture is evenly pink.
Place a bamboo rolling mat on your work surface. Lay one sheet of nori on the mat, rough side up, with a short end parallel to you.
Lightly dip your fingertips in cool water to prevent sticking.
Spread about ¾ cup / 150 g (5.3 oz) of the pink rice evenly over the bottom three-quarters of the nori sheet, leaving a border at the top.
Create a line of fillings across the center of the rice. Smear ½ teaspoon of grated ginger, sprinkle with ¼ teaspoon of lemon zest, and then arrange ¼ of the roasted beet strips, carrot matchsticks, and watercress horizontally.
Using your thumbs to lift the edge of the mat closest to you, roll it up and over the fillings. Use your other fingers to tuck the fillings in tightly as you roll.
Continue rolling forward, applying gentle, even pressure, until the roll is sealed. Be careful not to roll the mat into the sushi!
Gently shape the roll by pressing on the top and sides with the mat to create a firm, even cylinder.
Place the finished roll seam-side down on a cutting board to rest for at least 2 minutes.
Repeat steps 2-7 to make three more rolls.
To serve, dip the blade of a very sharp knife in water.
Slice each roll into 5 even pieces using a swift, gentle sawing motion.
Serve immediately with Ponzu Sauce for dipping.
Don't overfill your rolls! It's tempting to add more, but it makes rolling difficult and can cause the nori to split.
When rolling, tuck the edge of the nori tightly over the filling to create a firm center and prevent a loose, sloppy roll.
