- Place the red onion in a small bowl. Cover with ice water and soak for 5 mins. This is going to help take the raw edge off the onion. Drain well then set aside.
- Cut the fish into 1 cm cubes and transfer to a medium bowl. Season with ½ tsp sea salt, toss gently to combine and leave for 2 mins. This will help the fish firm up a little so it doesn't break when stirring in the rest of the ingredients.
- Meanwhile, combine the red onion, chilli, lime juice and orange juice in a small bowl; this is the marinade.
- Pour the marinade over the fish and toss gently to combine. Cover and leave to marinate in the fridge for 30 mins.
- Strain off the marinade, reserving the liquid. Add the cucumber, mango, fresh coriander and a drizzle of olive oil and stir gently to combine. Taste and adjust seasoning. You can add a couple of tablespoons of the marinating liquid if want it a little juicier. Divide the ceviche between bowls and serve immediately.