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Cook the pasta following pack instructions. Meanwhile, heat the oil in a medium non-stick pan. Add the leeks, stir well, then cover and cook for about 6 mins or until softened.
Put the flour and mustard powder in a small bowl and gradually whisk in the milk. Pour into the pan with the leeks and stir with a wooden spoon until thickened. Continue stirring over a low heat while the pasta cooks. It will seem thick but will be diluted later.
Drain the pasta, reserving the cooking water, then stir the paste into the sauce. Add the spinach and 1-11/2 ladlefuls of the water to make a spoonable sauce, then stir over the heat until the spinach has wilted. Stir in most of the cheese and taste for seasoning.
Heat the grill. Tip the pasta mixture into a casserole dish, arrange the tomatoes over the top and scatter over the remaining cheese. Grill for 5-10 mins or until bubbling and golden.