Filling: Heat the olive oil in a frying pan then cook and stir the onion until soft. Add the cooked, shredded chicken along with the olives and parsley. Stir well and season with salt and pepper. Remove from heat and set aside.
Pastry: Heat the stock and butter in a large saucepan over medium heat then add salt to taste. Once simmering, add flour all at once and stir to form a paste. Stir vigorously until the mixture is smooth and no longer sticks to the bottom of the pan. Remove from heat and let cool.
Once cool enough to handle, knead the pastry on an oiled work surface until smooth, for 2 to 3 minutes.
To Assemble: Take a portion of pastry and roll into a golf ball sized ball. Stick an oiled finger into the centre of the ball and press the edges to make a bowl out of the pastry. Stuff with 2 teaspoons of chicken filling and gather the top edges, sealing and rolling into a teardrop shape. Set aside and cover with a damp towel, then repeat with remaining pastry and filling.
Crumbing: Set out three bowls in the following order: plain flour, beaten eggs, breadcrumbs. Roll each croquette in flour, then dip in egg, then cover in breadcrumbs.
To Cook: Heat oil for deep frying in a large saucepan or deep fryer. Fry the croquettes, working in batches, for 7 to 10 minutes, or until golden brown on all sides. Remove from oil and drain on paper towel.