For the mayo, whisk the egg yolk and mustard together in a bowl. Slowly add the oil, a drop at a time and whisk continuously until the mixture starts to thicken and emulsify.   

Continue to whisk pouring the oil in at a steady drizzle until it has all been added. Whisk in the yuzu and season. Spoon into a piping bag. 

For the dip: grate the ginger into a bowl using a fine grater to extract all the ginger juice. Stir in the soy sauce and sesame oil.   

To make the sushi rolls, place a sheet or nori onto a sushi mat. Spoon about a quarter of the rice on top then use wet fingertips to flatten the rice down and cover the nori in a thin flat layer, leaving a 1cm border at the bottom and top of the sheet.  

Lay half the salmon horizontally along the bottom edge of the rice. Top with a quarter of the avocado, half the cucumber and half the coriander, pipe over the mayo and use the mat to wrap up and around the nori to roll it up tightly, wetting the edge of the nori sheet to help it stick down. Repeat with another nori sheet using the same quantity of rice cucumber avocado, coriander and the rest of the salmon.  

For the Californian rolls, place a nori sheet on top of a board. Spoon half the remaining rice on top and press down in a think flat layer, leaving a border as before. Sprinkle over half the sesame seeds and cover the rice with a sheet of clingfilm.   

Flip the whole thing over including the cling film onto the sushi mat. Place half the crab and half the remaining avocado at the bottom end of the rice, then roll up tightly using the mat and clingfilm. Wet the edge of the nori and secure. Repeat with the last nori sheet and remaining ingredients.  Slice the rolls into 6 pieces, trimming as necessary. Arrange on a plate and serve with the dip.