To start, Chef Andrew Nun palms and shapes a glob of rice equivalent in size to a baseball. It’s been cleaned evenly with rice vinegar — something very important — and cooked at a precise 140 degrees. He stomps the ball of rice out on seaweed paper, sprinkles on sesame seeds and adds cucumber, avocado and mock crab. He rolls it faster than one can say “California.” It must, he said, be fast.