I used puy lentils because they are what I had. They're my favourite. I sometimes use green lentils. I don't think I'd use red ones in this recipe. I forgot to mention that I fried a rosemary sprig alongside the fish. I'm not sure that it added that much, to be honest. I'm cautious with rosemary. It can be a bit of a hooligan if overused. I use it when roasting shoulder of lamb and when I do my Mediterranean-style potatoes for barbecues, in the latter case with as many unpeeled garlic cloves as you like. Nothing like sucking the sweet middles from the skins once they've had half an oily hour in the oven with the spuds.