Heat 3 tablespoons of the oil in a saucepan over a medium heat, add the shallots and garlic and sauté for a few minutes until soft.
Add the tomatoes, red pepper, coriander stalks, leeks, sweet paprika and chilli powder, season with salt and pepper and cook, stirring, for 2 minutes.
Pour in the stock and puy lentils and bring to a simmer for 5 minutes.
Add the cod, cover and cook for a further 8 minutes, until the fish is cooked through.
Mix the remaining oil in a bowl with the chilli flakes and coriander leaves, then stir in the lemon zest and juice.
Serve the stew in large bowls with the dressing drizzled on top.