For the kidneys, season the kidneys with salt and freshly ground black pepper. Heat the oil and butter in a frying pan over a medium heat and fry the kidneys for 5-6 minutes, turning occasionally, until golden-brown on all sides and cooked through.
For the sauce, place the cream into a small saucepan and bring to the boil. Reduce the heat, stir in the mustard and simmer until thickened. Season with salt and freshly ground black pepper and stir in the chopped chives.
To serve, place a piece of toast onto a plate, top with the kidneys and pour over the sauce.