Peel, wash and chop the potatoes and celery. Cook in boiling salted water for about 20 minutes. Peel, wash and dice the carrot. Peel and finely dice onion and garlic.
Heat the oil in a frying pan. Fry the minced meat in it in large pieces. Add onion, garlic and carrots and fry for about 5 minutes. Season with salt and pepper. Stir in chunky tomatoes. Drain potatoes and celery, mash coarsely.
Stir in butter. Grease a casserole dish. Add mashed potatoes, add chopped meat. Coarsely crumble feta cheese over it. Wash the thyme, pluck the leaves from the stalks and sprinkle over it. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 10-15 minutes.