Heat 50ml of the olive oil in a large frying pan over a medium heat and sauté the onions and garlic for 5 minutes until they start to soften.
Add the peppers and continue to saute for a further 8-10 minutes until the peppers are soft. Stir in the cooked rice and green olives and season well with salt, pepper and lemon juice. Make sure the rice is warmed through and keep it warm to one side.
Heat the rest of the olive oil in a large frying pan over a medium heat. Season the mackerel fillets and pan fry for 10-12 minutes, skin side down to begin with and turning halfway through. Arrange the warm rice in bowls and place the mackerel fillets on top.
Garnish with basil leaves and serve immediately.