Use scissors to cut the nori sheets into sixteen 10 x 4cm rectangles. Cut the cucumber, carrot and shallot into 6cm-long matchsticks. Cut the tuna crossways into 5cm-long strips.
Place 1 nori rectangle, shiny-side down, on a clean work surface. Place a piece of tuna along 1 narrow edge so the ends are exposed. Top with some pickled ginger, cucumber, carrot and shallot.
Lightly brush the end of the nori with a little water. Roll up firmly, pressing lightly to seal. Transfer to a serving platter and cover with plastic wrap. Repeat with the remaining nori, tuna, pickled ginger, cucumber, carrot and shallot, to make 16 rolls.
Top each roll with a small teaspoonful of salmon roe or caviar, if desired. Serve with the wasabi paste and soy sauce.