Place the pieces of fat into a small, heavy frying pan then switch on the heat to low-medium. Leave for the fat to render out, checking now and then that it isn't burning.

When no more more is being released (approx 20-30min), remove the remaining pieces of fat and pour the liquid fat into a suitable container for later use.

Place the sliced potatoes in a saucepan, add salt plus cold water to cover.

Bring to a boil, turn down to a simmer and cook until the potatoes are almost, but not quite done (8-10 min).

Strain the potatoes and leave to cool a little. You can also refrigerate at this point and saute the potatoes later.

Cut the cooled potatoes into chips (or cubes if you prefer).

Heat the rendered beef fat in a frying pan over a moderate-high heat then add the potatoes (in batches if necessary). Brown the potatoes on all sides, stirring around until they're cooked through and crispy on the outside. Drain on kitchen towel and keep warm while you cook the remaining potatoes.

Season with salt and pepper before serving with Rabo de Toro.