Preheat the oven to 200°C/gas mark 6. Bring a large pan of salted water to the boil and cook the pasta for 3 minutes less than the recommended time on the packet. Drain and add the oil, stirring it through, to stop the pasta sticking together.
Melt the butter in a small pan over a medium heat, then add the garlic and mustard powder and cook for 1 minute. Stir in the flour, and cook for another minute, mixing all the time. Add the milk and cream and whisk until the sauce is smooth and lump-free. Continue to whisk until the sauce thickens.
Take the pan off the heat. Add the grated cheddar and leave to melt, stirring occasionally.
Tip the pasta and kale into an ovenproof dish and pour over the sauce. Bake for 20 minutes until the top is crisp and golden. Sprinkle with the parmesan and season with black pepper to serve.