Cook the pasta for 8–10 minutes or according to packet instructions until tender. Drain and set aside.
Heat the butter in a nonstick pan until melted. Remove from the heat, add the flour and stir well to form a thick paste. Return the pan to the heat and cook, stirring continuously, for 1 minute. Remove from the heat and gradually add the milk, a little at a time, stirring well until all the milk is used.
Return the pan to the heat and bring to the boil, stirring continuously until boiled and thickened. Add the crème fraîche and half the Parmesan and stir well. Add the drained pasta, herbs, flaked salmon and peas and toss gently to mix, taking care not to break up the fish. Transfer to 6 individual gratin dishes and sprinkle over the remaining Parmesan.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 20 minutes until golden and bubbling. Serve with warm bread and a simple salad.