To make marinade, bring to boil rice vinegar, water, sugar and salt in a saucepan. Turn off the heat and set it aside to cool.
Sprinkle a pinch of salt and gently massage the sliced onion. Rinse the onion under cold water then squeeze excess water.
Once the marinade is cooled down transfer to a shallow dish large enough to hold all ingredients in 1 to 2 layers. Add onion and celery to the marinade.
Cut salmon into thin-one-bite size. Sprinkle the fish with salt and pepper.
Heat the frying oil to 375 fahrenheit. Dredge salmon in cornstarch and fry them in batches. Take them out on paper towels to drain excess oil. Repeat until all the fish are fried.
While the fish is still warm, add them to the marinade and coat them evenly with marinade. Sprinkle sliced lemon, cubed tomato, shichimi pepper, and sesame oil on top.
Marinate the fish at least three hours to overnight in the refrigerator until ready to serve.