Make the cheese sauce: Place the milk and reduced fat spread in a medium saucepan. Place over a medium heat and stir until the spread has melted and the milk is steaming hot. Take care not to let the milk boil over.
Mix the cornflour and water until smooth. Mix into the hot milk, stirring constantly with a wooden spoon or balloon whisk. Bring to the boil, reduce the heat and simmer for 3-5 minutes, stirring constantly until thickened and smooth.
Stir in the cheese, mustard powder and season well with salt and black pepper to taste. Cover, set aside and keep warm.
Prepare to poach the eggs by placing a large saucepan, filled with no less than 5cm (2in) of water on the stove and bring to the boil. Reduce the heat so that the water is just simmering.
Meanwhile, cook the spinach. Place the spinach in a colander or sieve and rinse under cold running water. Shake well to remove the excess water and transfer the spinach to a large saucepan. Place over a low heat and cook gently, stirring constantly, for 2-3 minutes, until wilted. Take care not to overcook; the spinach should be softened but still bright green.
Tip the spinach into a sieve and squeeze out as much liquid as possible, using the back of a wooden spoon. Return the spinach to the saucepan and stir in the reduced fat spread, ground nutmeg and season well with salt and black pepper, to taste. Cover with a lid and keep warm.
To poach the eggs, gently crack the eggs into the large saucepan of simmering water, making sure they are spaced apart. Immediately lower the heat; there should only be a few small bubbles in the water. If the water is bubbling too much this may cause the eggs to move around and break up.
Gently poach the eggs for 2-4 minutes, depending on how you like your poached eggs to be cooked. While the eggs are poaching, toast the muffin halves until light golden and spread each with a little reduced fat spread. Place the muffin halves on four plates.
Divide the warm spinach between the four muffin halves, spreading out roughly over the muffins.
Remove the poached eggs from the cooking water using a large, slotted spoon, allowing any water to drain away. Place a poached egg on top of the spinach.
Pour over the warm cheese sauce and season with salt and black pepper. Serve at once.