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Heat oven to 200C/180C fan/gas 6. Boil the potatoes for 10 mins until just tender, drain and steam-dry for a few mins. Toss the potatoes in the oil and butter, then tip onto a baking tray. Roast for 20 mins. Push the potatoes to one side and put the salmon on the other side, and add the lemon halves. Season. Return the tray to the oven for 15-20 mins until everything is cooked through – check the centre of the salmon with the tip of a knife.
Cook the peas in simmering water for 2 mins, drain and tip into a bowl. Add the crème fraîche and herbs, if using. Break the salmon into chunks to serve and squeeze over the lemon. Serve the salmon with the potatoes and peas, and some extra crème fraîche on the side, if you like.