Heat butter and oil in a saucepan and fry leeks for 4 minutes until softened.
Stir in the flour gradually and cook for 5 minutes.
Add milk gradually and cook for 15 minutes while stirring regularly.
Stir in the grated cheese, peas, seasoning, nutmeg, and parsley until the cheese melts.
Remove from heat, cover with cling film, and allow to cool completely.
Beat eggs in a bowl and place breadcrumbs on a separate plate.
Heat vegetable oil in a deep pan or fryer to 180°C.
Roll the cooled mixture into 7.5cm cylinders using floured hands.
Dip croquetas into the beaten egg and roll in breadcrumbs to coat.
Fry for 4 minutes until golden-brown, then drain on kitchen paper.
Mix chimichurri ingredients in a bowl and serve with the croquetas.