This is nice with crusty bread to mop up the sauces. It's nice & light with lots of flavours & could easily be reduced to serve as an entree instead of a main. Cod is a favourite in Britain - you can easily buy it frozen all year round.
Heat the oil in a large pan (that has a lid) on medium heat.
Add the onion, celery & garlic & saute for 3-4 minutes, taking care not to burn.
Add the bacon & saute for 4 minutes.
Add the puy lentils & stir to coat in the oil.
Add the chopped tomatoes and bay leaves, stir and increase the heat until the liquid is boiling. Boil for 2 minutes.
Add the stock, reduce the heat so the mixture is simmering, then cook for 30 minutes, or until the lentils are al dente.
When the lentils are nearly done (I reckon this is about 10 minutes from the end - they will taste a bit soft but still retain a slight firmness) place the cod fillets on top of the lentil mixture, pressing slightly into the mix, squeeze the lemon over the top and put the lid on your pan. Cook until the fish flakes easily with a fork.