1.Make romesco sauce. Soak chilli flakes in hot water for 5 minutes; drain. Blend or process chilli, garlic, nuts and tomato until smooth. With motor operating, gradually add combined oil and vinegar in a thin steady stream until sauce is smooth.
2.Blend or process chorizo, onion and garlic, pulsing until ingredients are finely chopped and combined. Transfer mixture to a large bowl; stir in beef, nutmeg, sherry and breadcrumbs; season. Roll level tablespoons of the mixture into balls.
3.Heat oil in a large frying pan; shallow-fry meatballs, in batches, until browned and cooked through. Drain on absorbent paper.
4.Serve meatballs with romesco sauce.
Meatballs and sauce can be made a day ahead. To reheat the meatballs, place them in a single layer, on an oven tray; cover with foil, making several slashes in the foil to allow steam to escape. Bake in an oven preheated to 180°C/350°F for about 20 minutes.