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 crab and poached eggs.