Begin by trimming and halving the lamb kidneys. Remove any membrane and the white fatty core, as these can cause bitterness. Pat dry with kitchen paper to ensure even browning.

In a shallow dish, combine the flour, mustard powder, cayenne, salt, and black pepper. Toss the kidneys in this seasoned flour until evenly coated. Shake off any excess and set aside while you heat the pan.

Heat the butter and oil together in a large frying pan over medium heat. Once the butter foams, add the chopped shallot and cook gently until softened and translucent. This forms the aromatic base of the sauce.

Increase the heat slightly and add the floured kidneys to the pan. Sauté for about 2 minutes on each side until golden brown but still tender within. Avoid overcooking, as kidneys can become tough.

Remove the kidneys from the pan and set aside on a warm plate. Add Worcestershire sauce, Dijon mustard, and tomato purée to the pan juices. Stir briskly to loosen any browned bits stuck to the bottom of the pan.

Pour in the beef stock and stir to combine. Simmer for 3 to 4 minutes until the sauce thickens slightly and becomes glossy. Taste and adjust seasoning if needed, balancing the spice and tang.

Return the kidneys to the sauce, coating them well. Cook gently for 1 to 2 minutes to heat through. If desired, stir in a splash of double cream for a smoother finish.

While the kidneys finish, toast thick slices of crusty bread until golden and crisp. Butter them lightly to help soak up the rich sauce.

Spoon the devilled kidneys over the toast and drizzle with any remaining sauce. Scatter freshly chopped parsley on top for a burst of freshness.

Serve hot for breakfast or brunch. The dish pairs wonderfully with grilled tomatoes or sautéed mushrooms for a full English style plate.