Butter a large circular gratin or pie dish (approx 10' diameter). Pre-heat the oven to 180°C / gas mark 4.
Bring a large pan of lightly salted water to the boil and cook the spaghetti until just al dente, you still want a bit of bite to the pasta. Drain well, turn into a large bowl and toss with a little butter.
Make the béchamel sauce, following this method.
Spoon half the spaghetti into the baking dish and cover with the smoked salmon. Cover with half the béchamel sauce and sprinkle with parmesan. Add the remaining spaghetti on top and cover with the remaining béchamel sauce. Sprinkle with the remaining parmesan and dot with butter.
Place the dish in the middle of the pre-heated oven and bake for 15-20 minutes until the top is nicely browned. Serve immediately.