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 together 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. Turn off the heat and cover to keep warm.
Meanwhile, place the spinach into a large saucepan. Place over a low heat and cook gently, stirring constantly, for 2-3 minutes, until wilted.
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. Turn off the heat and cover with a lid to keep warm.
To poach the eggs, place 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, 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.
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 lightly 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.
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 pepper. Serve at once.