Cook the spaghetti in a large saucepan of boiling water with the salt and oil for 8–10 minutes, or according to the packet instructions, until al dente.
Melt the butter in a saucepan, sprinkle in the flour and cook over a moderate heat, stirring, for 1–2 minutes. Gradually pour in the stock, beating vigorously after each addition. Bring to the boil, stirring, then reduce the heat and simmer, stirring, for about 5 minutes until thick and smooth.
Remove from the heat, stir in the cream and mustard, and season to taste. Gently fold in the turkey strips.
Drain the spaghetti and spread half in the bottom of a lightly greased ovenproof dish. Cover with half the turkey mixture, then the remaining spaghetti. Top with the remaining turkey and sprinkle with the Parmesan.
Place in a preheated oven, 200°C (400°F), Gas Mark 6, for 10 minutes or until golden.