Finely chop the red pepper and spring onions. Place the cooked brown rice in a large mixing bowl and add the chopped vegetables.
Whisk together the olive oil, lemon zest and juice, cumin, oregano and garlic, if using. Pour the dressing over the rice and toss gently to combine.
Blanch or steam the frozen green beans or edamame beans for about 5 minutes, or until just tender. Drain and allow to cool.
Drain and rinse the tinned beans.
Remove the skin from the smoked mackerel and break the flesh into bite-sized flakes, checking carefully for any bones.
Add all the beans, sweetcorn and mackerel to the rice mixture. Fold everything together gently, taking care not to break up the fish too much.
Taste and season with salt and freshly ground black pepper. Scatter the chopped parsley over the top and serve.