Wash pork fillets, dab dry, cut into 1.5 centimetre wide slices. Heat oil in a pan, fry meat briefly on both sides, season with salt and pepper.
Remove meat from the pan and keep warm. Carve tomatoes crosswise, blanch with hot water in ice water, remove skin. Cut tomatoes in half, remove seeds and cut into small cubes.
Peel and finely dice the onion and garlic cloves and fry them in the pan. Add the diced tomatoes. Let everything boil down until almost all liquid has evaporated. Cut olives into fine slices.
Add olives and sherry to the pan. Season to taste with salt, pepper and sugar. Add medallions of pork and let everything simmer for ten minutes at low heat, covered. Serve garnished with parsley.