Melt butter in a large saucepan, whisk in flour for 3 minutes, then gradually add milk to make a thick sauce.
Season with salt and pepper, stir in fresh breadcrumbs, chicken, and tarragon, then chill in the fridge for 4 hours.
Shape the mixture into 7cm cylinders.
Coat each cylinder in flour, then beaten egg, and finally a mix of panko breadcrumbs and ground almonds.
Preheat rapeseed oil in a deep-fat fryer to 170°C.
Fry croquetas in batches until golden-brown and drain on kitchen paper.
Whisk egg yolks with mustard, then slowly whisk in oil to create mayonnaise.
Stir in vinegar, piquillo peppers, and parsley, then season.
Serve croquetas with watercress and the pepper mayonnaise.