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 sushi vinegar (or the mixture of rice vinegar, sugar and salt) and mix well.
Poach Shrimp: Bring a pot of water to a boil, then sprinkle a pinch of salt. Add shrimp, cover the pot and remove from heat. Let stand until shrimp are cooked through, about 3-5 minutes. Transfer poached shrimp to a bowl of ice water to stop the cooking process. Drain, peel the shrimp, and remove the tails.
Make Boston Sushi Roll: Lay out the bamboo mat with a piece of plastic wrap on top (this will make clean 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 towards the bottom of the mat.
Lightly wet your hands and take about 3/4 cup of cooked rice. (You can dip your hands in vinegar water to prevent sticking*.) Gently spread the rice out to the edges in a thin even layer. Don’t use too much pressure, or you’ll end up with mushy rice.
Flip the rice and nori over so that the rice is on the bottom and the nori is facing up.
Place shrimp, avocado, and cucumber on top of the nori. Make sure not to add too much fillings or your roll won’t seal properly.
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.
Remove the bamboo mat, and spread tobiko on to 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 bamboo mat while keeping the plastic wrap. Slice the roll into 8 bite-size pieces. Remove the plastic wrap from each piece. Serve and enjoy!