Cook the fish: preheat the grill. Mix the lemon zest with one tbsp of oil. Line a roasting tin with foil, oil lightly and add the fish. Brush the fish with the lemon oil. Season and grill for 8-10 minutes until cooked (no need to turn it).
Prepare the chickpeas: meanwhile, heat the rest of the oil and fry the onion until golden. Add the spinach and cook until wilted. Stir in the chilli flakes. Tip in the chickpeas and one tbsp of lemon juice, heat through and season. Spoon the chickpea mixture on to hot plates and serve the fish on top. Drizzle with a little extra oil or lemon juice.