Cut each tenderloin crosswise into 2 cm thick slices. Season with salt and pepper. Heat the oil and butter in a pan over medium heat. Add the pork slices then sauté until golden-coloured, about 4 min per side. Take them out of the pan and set aside on a plate, keeping them warm in the oven.
Add the garlic and cayenne pepper to the pan, then cook 1 min with stirring. Add the sweet potatoes andoregano, then sauté 2 min with stirring. Pour in the grapefruit juice and water, then bring to a boil. Lower the heat, cover and simmer until the potatoes are tender, about 12 min.
Put the pork slices back into the pan then heat them through, 2-3 min. Sprinkle with pine nuts (optional) and chopped mint, then serve.