Add the butter and 2 tbsp olive oil to a saucepan over a low heat and allow to melt. Add the spring onions and sweat for 1 minute. Whisk in the flour and cook out the flour for 8 minutes.
Gradually whisk in the hot milk, heat until the mixture thickens to the consistency of creamy mashed potato. Remove from the heat and whisk in the cheese, crab and nutmeg, season to taste. Place in a bowl, seal the top of the béchamel with a layer of cling film making sure the cling film covers the surface of the béchamel. Place in the fridge for 2 hours or until completely chilled.
Use floured hands to roll spoonfuls of the béchamel into cylinders, dip in the egg and then roll in breadcrumbs. Heat the olive oil to 180ºC and fry the croquetas for a couple of minutes until golden brown. Serve with a green salad, tartare sauce and a wedge of lemon.