Peel and finely chop the onion. Heat 1 teaspoon of oil in a saucepan and sauté the onion. Add rice. Stir in tomato paste. Add stock, bring to the boil and simmer at low heat for about 20 minutes.
Wash and clean the tomato and cut 2 slices from the middle. Finely dice the rest of the tomato. Wash the thyme, dab dry and remove the leaves. Dab pork medallions dry and season with salt and pepper.
Heat 1 teaspoon of oil in a frying pan. Fry the medallions for about 3 minutes, turning them over. Cover with 1 slice of tomato and 1 slice of mozzarella. Sprinkle with thyme. Cover and fry on a low heat for another 3 minutes until the cheese has melted.
Fold the tomato cubes into the rice. Season to taste with salt and pepper. Arrange the rice and pork medallions on plates with some roast. Garnish with thyme. Serve with a green salad.