Cook Sushi Rice: Wash the rice and add it to the rice cooker together with water. Once cooked, transfer to a large bowl and let it cool down slightly. When it’s still very warm, stir in the optional sushi vinegar (or the mixture of rice vinegar and sugar).
Make Tobiko Sushi: Lay out the bamboo mat with a piece of plastic wrap on top (this will make cleaning up easier and prevent rice from sticking to the bamboo).
Fold the nori sheets in half and split them using a pair of scissors.
Place half of the nori sheet on top of the bamboo.
Take 3/4 cup of cooked rice and evenly spread it over nori. (You can dip your hands in vinegar water to prevent sticking.)
Flip everything so that rice is facing down.
Place shrimp, avocado, and cucumber on top of the nori.
Place the thumbs underneath the bamboo mat and lift the edge up and over the filling.
Roll the bamboo mat away from you and apply some pressure to tighten it up. Keep rolling until the ends meet.
Add Tobiko Topping: Remove the bamboo mat, and spread tobiko on top of the roll.
Place plastic wrap on top, and cover with the sushi mat. Squeeze gently to press the tobiko around the roll.
Remove the mat while keeping the plastic wrap. Slice the roll into bite-size pieces. Remove the plastic wrap from each piece. Serve and enjoy!