1. Place rice in a bowl, cover with water and soak for 30 mins to 1 hour.
2. Drain and rinse rice several times till the water runs almost clear. Drain rice completely.
3. Place rice in a rice cooker with the 425ml of water and cook according to rice cooker instructions.
4. Place salt, sugar and rice vinegar in a small bowl. Stir to mix until most of the sugar is dissolved.
5. As soon as the rice is cooked, transfer to a large mixing bowl.
6. While the rice is still steaming hot, drizzle the sugar mixture over the rice and stir with a fork to distribute the mixture evenly.
7. Cover bowl with a clean dish towel and leave the rice to cool completely before using.
8. To make the mentaiko mayo, squeeze the roe out of the mentaiko sac into a medium-sized bowl. Discard the sac. Add the Japanese mayo and stir to mix. Transfer to a squeeze bottle or a piping bag. Set aside
9. To assemble, place a sheet of nori, shiny side down, on your work surface. Using the tip of a sharp knife or a pair of scissors, cut a slit from the centre of the sheet to the left of it.
10. Spread a square of rice on the top left quarter of the nori sheet. Do the same on the bottom left half. Be sure to leave a border around both squares of rice.
11. On the top right quarter, arrange the sliced salmon to form a square. If your mentaiko mayo is in a piping bag, snip the corner of the bag and pipe the mayo in a zigzag pattern over the salmon.
12. On the bottom right quarter, arrange the slices of avocado to form a square, again leaving a border around it.
13. Using a blow-torch, gently torch the mentaiko mayo on top of the salmon till you get a light layer of char. Be careful as the oils from the salmon will sputter as they are torched.
14. Fold the nori from the top left quarter of rice over and on top of the top right quarter of salmon.
15. Now fold downwards to the bottom right, over the avocado, then fold towards the bottom left. You’ll now have a sushi sandwich of sorts. Press down the sides to seal and slice into half, if you like.
16. Repeat with the rest of the rice and ingredients. Serve immediately, with shoyu and wasabi on the side.