Step 1

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.

Step 2

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.

Step 3

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.

Step 4

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.