Wash sage, shake dry and remove leaves from 2 stems, chop finely. Knead butter and chopped sage and season with salt and pepper. Form into a roll, wrap tightly in cling film and chill.
Wash the tomatoes, potatoes and unpeeled garlic cloves. Cut the tomato panicles in half. Dab meat dry and season with salt and pepper. Heat 1 tablespoon of oil in a frying pan. Brown the pork fillet briefly all around, then place on a baking tray.
Halve or quarter the potatoes depending on size. Spread potatoes and garlic around the meat. Season with salt and pepper and sprinkle with remaining oil. In a preheated oven (electric cooker: 200 °C/ circulating air: 175 °C/ gas: see
manufacturer) bake for about 30 minutes.
Cut butter into slices. Remove meat from the tray about 10 minutes before the end of cooking time. Spread about half of the sage butter on the meat. Cover the meat with aluminium foil and let it rest. Place the tomatoes and the rest of the sage butter on the baking tray and finish cooking.
Cut the meat into slices and put it back on the tray. Garnish with remaining sage and serve.