Heat 1 tablespoon of the butter and 1 tablespoon of the oil in a saucepan. Add the shallots and lardons and cook over a low heat for 7–10 minutes until very soft. Stir in the peas and stock and simmer for about 5 minutes. Stir in the crème fraîche and season with salt and pepper. Roughly chop the lettuce and toss around the pan for 1 minute to coat in the juices.
Meanwhile, heat the remaining oil in a frying pan. Pat the fish dry with kitchen paper and season with salt. Cook for 5 minutes on each side until just cooked through. Add the remaining butter to the pan along with the lemon juice and swirl around until the butter has melted.
Serve the fish with the peas and lettuce together with some new potatoes, if liked.