Cut cucumber lengthwise into thin strips. Discard the core (seeds).
Cut the avocado in half lengthwise around the seed and twist the two halves until they separate. Hack the knife edge into the pit. Hold the skin of the avocado with the other hand and twist in counter directions. The pit will come out smoothly.
Remove the skin and slice the avocado width wise with the tip of knife.
Gently press the avocado slices with your fingers and then keep pressing gently and evenly with the side of knife until the length of avocado is about the length of sushi roll (length of nori seaweed). Squeeze lemon over the avocado to prevent from changing color.
Wrap the bamboo mat with plastic wrap and place half of the nori sheet, shiny side down. Dip your hands in tezu (vinegar water) and spread the rice over the nori sheet.
Turn it over and put the shrimp tempura, cucumber strips, and tobiko at the bottom end of the nori sheet. If you like to put unagi, place inside here as well.
From the bottom end, start rolling nori sheet over the filling tightly and firmly with bamboo mat until the bottom end reaches the nori sheet. Lift the bamboo mat and roll over.
Place the bamboo mat over the roll and tightly squeeze the roll.
Using the side of knife, place the avocado on top of the roll.
Place plastic wrap over the roll and then put bamboo mat over. Tightly squeeze the roll until the avocado slices wrap around the sushi. Be gentle so you won’t break the avocado slices.
Cut the roll into 8 pieces with knife. Clean knife with wet towel after each cut. Remove the plastic wrap on top of the sushi and transfer to a serving plate.
Put tobiko on top and drizzle spicy mayo and unagi sauce. Sprinkle black sesame seeds if you like. Enjoy!