Preheat oven to 350° F and bring water to a boil
Lightly grease a baking tray with a small amount of vegetable oil.
Mix the soy sauce, ginger and wasabi in a dish that will fit all four salmon fillets. You can use a shallow dish similar to casserole dish. Place the salmon flesh side down and let it marinade for 15 minutes.
Once the water is boiling, add in the rice, lower the heat, cover and let it simmer for 15 - 20 minutes or until tender.
Transfer salmon fillets to the prepared baking tray, skin side down. Bake the salmon for 10-15 minutes, or until easily flaked with a fork.
Make your kewpie mayonnaise by whisking together the mayonnaise, rice vinegar, and sugar.
Wash and prep the green onions by slicing them diagonally and set aside for garnishing.
Spread the rice between four dishes and serve the baked salmon fillets on the bed of rice. Drizzle with the kewpie mayonnaise (homemade or store bought), and garnish with the sliced green onions. Enjoy!