Heat half the oil in a frying-pan. Add the kidneys and seasoning and fry until they are cooked through and tender.

Using a slotted spoon, transfer the pieces to a plate. Keep warm.

Add the onion and garlic to the pan and fry until they are soft. Remove from the heat and keep hot.

Heat the remaining oil in a small saucepan. Remove the pan from the heat and stir in the flour to form a smooth paste. Gradually add the stock, stirring constantly. Return the pan to the heat and bring to the boil, stirring constantly. Cook for 2 minutes, stirring constantly, or until the sauce is smooth and thick. Stir in the reserved onion and garlic, reduce the heat to low and simmer for 3 minutes. Remove the pan from the heat.

Return the frying-pan to the heat, add the sherry and bring to the boil. Add the kidneys and onion sauce and bring to the boil, stirring constantly. Reduce the heat to low and simmer for a further 5 minutes.

Transfer the mixture to a warmed serving dish and serve at once.