Heat a large, deep frying pan to a medium heat and add 2 tablespoons of butter. Add the chopped leeks, season well with salt and pepper, and cook for 5 minutes until the leeks are soft. To add a more lemony flavor, add 1 teaspoon of lemon zest at this point.
Add the spinach to the frying pan and allow it to wilt then mix in with the leeks. Remove the spinach and leeks from the heat and set to one side.
To make the hollandaise sauce, place the remaining butter into a heatproof bowl or jug and melt it in the microwave, then set to one side.
In a medium-sized bowl, whisk together the three egg yolks with the lemon juice and 1 tablespoon of water for a few minutes, until the mixture is frothy and doubled in size.
Set the bowl over a saucepan of boiling water and continue to whisk for 4 to 5 minutes until the mixture begins to thicken.
Remove the bowl from the saucepan and set on a solid surface. Whisk continually while gradually pouring the melted butter into the bowl. Keep whisking until all of the butter is incorporated and the mixture is glossy. Add a pinch of cayenne pepper and whisk it in.
To poach the eggs, simmer water in a small saucepan. Break an egg into a small bowl or jug, and then use a wooden spoon to gently stir the water for 10 to 15 seconds, making a whirlpool. Carefully drop the egg into the centre of the whirlpool and allow the egg to poach for 2 to 3 minutes, until it is firm and white on the outside but still soft and liquid in the centre. Scoop out the poached egg using a slotted spoon and gently set it down onto a plate covered with a sheet of kitchen towel.
To assemble the eggs Florentine, first toast the English muffins. Scoop spoonfuls of the spinach and leek mixture onto the warm English muffins, and then top with the freshly poached eggs. Finish off by spooning a few scoops of hollandaise sauce on top, and serve warm.