Heat the olive oil in a pot and fry the leek, carrots, celery and garlic over medium-low heat for 5 minutes.
Stir in the lentils and stock and bring to the boil. Reduce to a simmer, partially cover the pot and cook for about 30-40 minutes for brown lentils and around 20-30 for Puy lentils.
Check the lentils are cooked through and season to taste.
Meanwhile prepare the cod. Preheat the oven to 220C (425F), line a large tray with baking paper and drizzle with a little olive oil.
Pat the cod dry with some kitchen paper then wrap in the pancetta slices.
Place the cod fillets on the prepared tray and scatter the tomatoes and basil leaves around them. Drizzle with a little olive oil and season well with salt and pepper.
Reduce the heat to 200F (400F), place the tray in the middle oven shelf and roast for 20 minutes or until the cod is firm and cooked through and the tomatoes blistered.
Pour any juices from the tomatoes in with the lentils. Divide the lentils and tomatoes between four plates and place the cod over them. Garnish fresh oregano and serve.