Cook the pasta in plenty of salted boiling water for about 10 minutes until just tender. Drain and return to the pan.
Meanwhile heat the oil in a large frying pan on the hob and gently fry the onions for about 10 minutes, stirring frequently, until beginning to brown. Add the thyme and garlic, and fry for a further 1 minute. Add to the drained pasta along with half the béchamel sauce and cheese.
Turn half the mixture into a shallow ovenproof dish and scatter the beetroot slices on top. Tip the remaining pasta over the beetroot and spoon over the remaining sauce. Sprinkle with the remaining cheese and place on the upper rack of the halogen oven.
Set the temperature to 200°C (392°F) and cook for 15–20 minutes until the sauce is bubbling and turning golden. Serve garnished with thyme.