Fill a large, shallow pan with water, about 3cm (1in) deep and bring to a simmer. Add some vinegar (1-2 tablespoons per one litre of water) to help the eggs hold their shape.
Break four eggs into separate coffee cups (this is also to help eggs maintain their shape when they're placed in the pan). Gently tip the eggs into the simmering water and poach for 2-3 minutes.
Slice the muffins in half and lightly toast.
Once the eggs are poached, remove with a slotted spoon and drain on paper towels.
Place the spinach on top of the muffins, top with the egg and spoon a generous amount of hollandaise sauce over the eggs.
Serve at once.
