Dice the red onions and the peppers, and thinly slice the garlic. Cut the potatoes into small cubes and fry them for 15 minutes in 140°C oil. Season the tenderloin with salt and pepper, then fry on all sides in the oil and finally roast for 12 minutes in an oven preheated to 160°C. Fry the onions, tomatoes and peppers in a little oil, then add the fried potatoes, mix and sauté together for another 5 minutes. Cut the tenderloin into slices and serve on the bed of potatoes, sprinkled with finely chopped parsley.