To make teriyaki sauce, combine soy sauce, pineapple juice, brown sugar, mirin, and ginger in a small pot. Bring to a gentle boil over medium heat stirring occasionally.
Mix cornstarch with 1-1/2 tablespoon water and add to the sauce. Cook until he sauce thickens, about 2 minutes constantly stirring. Remove from the heat and set aside to cool. The sauce will thickens more as it cools.
To make Sushi rice, rinse the rice 5-6 times to remove starch coating from the grain until the water seems clear. Add 2-1/2 cup water and a piece of dried sea kelp and let it soak for 10 minutes.
Remove the sea kelp and bring the rice to boil over high heat. When the rice boils, stir with a folk to evenly distribute the grain. Reduce the heat to low and cover loose with a lid, simmer for 3 minutes. Cover tight with a lid and continue to simmer for 10 more minutes. Turn off the heat and let the pot sit for another 5-10 minutes.
Meanwhile mix vinegar, sugar and salt in a small microwave-safe bowl. Microwave for 30 seconds. Stir well.
Transfer the rice into a wooden or glass mixing bowl. Add the vinegar solution to the rice and mix in a folding motion to coat all the rice grain with the vinegar solution. Cover with a clean cloth and let the rice cool to the room temperature.
To make beef steak, season the steak with salt and pepper lightly. Grill or pan fry the steak by searing all the surface first. If the steak is too thick, you will need to finish it off in the oven. Cook the steak to medium doneness. Let the steak rest for 5 minutes.
Slice the steak against its grain, between 1/8 to 1/4-inch thickness.
To assemble the beef steak sushi, soak the onion slices in ice cold water for 5 minutes, drain and spin off to remove the excess water. Seat aside.
Take 2 tablespoon of sushi rice and press gently to shape into an elongated oval form. Dot with a small amount of wasabi on the top.
Place each sushi on a large serving platter. Top with a slice of beef steak, a couple of onion slices, and spring sprouts. Drizzle teriyaki sauce over. Enjoy!