For the mackerel, place the mackerel fillets onto a grill pan or flat tray. Mix the remaining ingredients together and brush over both sides of the fillets. Set aside to marinade for 30 minutes.
To cook the mackerel, heat the grill to its highest setting. Grill the fillets for two minutes on each side, basting with the marinade as you turn them.
For the lemongrass noodles, cook the egg noodles in a large pan of boiling water according to the packet instructions then drain, refresh in iced water and drain again thoroughly. Toss the noodles with half of the olive oil and set aside.
Heat a frying pan until medium hot, add the remaining olive oil, lemongrass, garlic, ginger, chilli and spring onions and cook gently for 1-2 minutes, or until the vegetables have just softened. Mix the noodles into the vegetables and stir in the Chinese leaf.
Whisk the oyster and soy sauces with the sesame oil and lime juice, then mix into the noodles and season to taste with salt.
To serve, pile the noodles onto each plate, top with the grilled mackerel and serve garnished with chopped coriander.