Cook the spinach for 5 minutes with water and salt. Drain thoroughly in a colander.
Melt the butter in a saucepan on a low heat. Add the spinach and flour. Stir until the spinach absorbs the flour.
Gradually add the milk into the saucepan and keep stirring until it forms a green and thick bechamel sauce. Add the blue cheese and stir until the cheese melts.
Remove the saucepan from the heat and leave the mixture to cool a little.
Divide the mixture into portions and shape them into croquettes using your hands.
Dip the croquettes into the whisked egg and breadcrumbs.
Spray the croquettes with oil and cook in the fryer until they turn golden. Turn them over halfway through cooking.