Prepare Whole30 sushi rice: Toss steamed cauliflower rice with tapioca starch. Heat liquids in a small saucepan until hot, then pour over cauliflower rice and stir well. Spread mixture on paper towels and let cool completely. Press with a paper towel on top to remove excess moisture.
Prepare your green onions: thinly slice half of one dark green part of green onion. Finely mince the remainder.
Stir together chopped salmon, 1/4 tsp. sesame oil, sriracha, and minced green onions.
Make the spicy mayo: in a small bowl, stir together both ingredients. Transfer to a small sandwich bag.
Make the ginger-sesame dipping sauce: combine all ingredients in a blender and blend until very smooth.
Roll your sushi: place 1 sheet of seaweed on your bamboo mat, shiny side down, or in your sushi maker. Press half of the cooled cauliflower rice onto seaweed in a thin layer, leaving a 1/2 inch edge at the top and bottom uncovered. About 1 inch away from the bottom edge of the seaweed, arrange cucumber, avocado, red cabbage, and smoked salmon mixture. If using a sushi maker, you might want to place the toppings in the center; follow manufacturer's directions.
If using a bamboo mat, cover the toppings with the edge of the sushi closest to you. Tightly roll with the bamboo mat. If using a sushi maker, follow manufacturer's directions. It might not be possible to extrude the sushi, if that's the type of maker you have, but you can use the maker to shape the sushi.
Slice into about 6-8 pieces with a very sharp knife. It can help to wet the knife in between slices.
Arrange sushi slices flat-side down and snip the edge off the spicy mayo in the sandwich bag. Drizzle spicy mayo on top and sprinkle with green onions and sesame seeds. Serve with ginger-sesame dipping sauce.