Juice the lemon and orange into a large bowl. Stir in 1 tsp caster sugar and a pinch of salt. This is your ceviche dressing.
Peel and finely slice the red onion. Add it to the ceviche dressing.
Halve the avocado. Scoop out the stone. Scoop the avocado out of the skin. Dice it and add it to the dressing. Set aside for 20 mins to marinate.
While the veg marinates, tip the quinoa into a sieve and rinse it under cold water for 1-2 mins. Tip the quinoa into a small pan. Pour in 150ml boiling water. Cover.
Put the quinoa pan over a high heat. Bring to the boil. Turn the heat down and simmer for 10-15 mins till the quinoa has absorbed the water and is tender.
While the quinoa cooks, heat the grill to high. Line a baking tray with foil. Rub 1 tsp oil and some salt and pepper into each cod fillet. Lay them on the tray, skin-side up, and grill for 10-12 mins till the skin is crisp and the fish is opaque and flakes easily when you press a fork into it.
Separate the lettuce leaves from half the lettuce (see our tip for ideas on what to do with the other half) and arrange the on two warm plates. Top
them with the quinoa and the cod fillets. Spoon over the ceviche dressing, the red onion and the avocado. Garnish with coriander leaves to serve.