Place the kipper fillets into a pan (or heatproof bowl) and pour boiling water over them. Leave for 10 minutes until the flesh comes off the bones fairly easily. Carefully remove the small bones.
Make the hollandaise sauce by melting the butter in a small pan. Put the yolks in a heatproof bowl over a pan of simmering water, slowily beating in the butter with a whisk. Season with lemon juice and salt and then remove from the heat. Whisk the sauce regularly to stop it from separating.
Halve and toast the muffins. Poach the eggs in a pan of simmering water. For the spinach; wilt gently until cooked, season to taste.
Top the muffins with the spinach, hollandaise sauce, flaked kipper and poached eggs.