Cook the rice following pack instructions, adding the soya beans to the pan for the final 3 mins.
Meanwhile, mix together the teriyaki sauce, garlic, lemon zest and juice. Heat a non-stick frying pan over a medium-high heat. Brush a little of the teriyaki sauce mixture over the mackerel and sear the fillets, skin-side down, for 1-2 mins. Turn the fish over and add the teriyaki sauce mixture to the pan, bring to a bubble and cook the sauce for 1-2 mins more. The fish will warm through and the sauce will thicken.
Spoon the rice into bowls, then top each bowl with a mackerel fillet. Spoon over the sauce and sprinkle over the spring onions. Serve with a blob of wasabi paste, if you like.