Peel and finely chop the onion. Peel and grate or crush the garlic. Grate or pare the zest from the lemon. Keep the lemon for later.
Warm 2 tsp oil in a deep frying pan or wok. Add the onion. Season with salt and pepper. Stir and fry over a medium heat for 5 mins till the onion is soft and glossy. Add a splash of water if it starts to stick.
Add the garlic, lemon zest and a pinch of dried chilli flakes (they’re spicy, so add as much or as little as you like). Stir and fry for 1 min.
Add the chopped tomatoes to the pan. Pour in 200ml boiling water. Add half the marjoram sprigs. Pop on a lid. Simmer for 10 mins till the sauce has started to thicken. Stir every so often.
While the sauce cooks, slice the thick cores out of the kale leaves. Finely shred the leaves into ribbons.
Add the cod fillets to the tomato sauce. Cover. Simmer for 10-12 mins till the cod is opaque and cooked through – the flesh should flake when pressed with a fork.
Add the kale to a pan with 1 tsp oil and 4 tbsp water. Cook and stir over a medium heat for around 5 mins till just wilted.
Drain the kale well and divide between 2 warm plates. Top with the cod and tomato sauce, discarding the marjoram sprigs. Sprinkle over a few marjoram leaves and serve with lemon wedges for squeezing.