Preheat an oven to 180 degrees/gas 4. In a bowl, mix together the leeks and frozen peas along with a good glug of olive oil and the lemon juice. Season lightly and then put this mixture on the bottom of a gratin dish. Top with the salmon fillets and spoon over the Greek yoghurt and herbs. Season the yoghurt topping and loosely cover with foil. Bake for about 20 minutes. Be careful when you peel back the foil as you can get a nasty burn from the steam. It won’t look as appetising as when you started, colour-wise, but it’s a lovely dish that is incredibly easy to prepare.