Wash the zucchini and cut off 1/4 to 1/3 of the zucchini horizontally above the stem base. Hollow out the zucchini. Cut the zucchini segments into fine cubes. Peel and wash the potatoes and cook them with the hollowed out zucchini flesh in boiling salted water for about 20 minutes.
Cook the hollowed zucchini in plenty of boiling salted water for 8-10 minutes. Add the courgette cubes in the last 1-2 minutes and cook along. In the meantime peel onions and garlic. Cut 2 onions into rings, remaining onion into cubes.
Press the garlic through a garlic press. Wash the thyme, dab dry and remove the leaves from the stalks, except for a few for garnishing, and chop. Heat olive oil in a pot, fry diced onion and garlic until transparent.
Add tomato sauce and thyme, season with salt and pepper and bring to the boil. Simmer sauce at low heat for about 5 minutes. Drain whole zucchini and dices on a sieve. Warm the milk and drain the potatoes.
Add milk and 1 tablespoon of fat to the potatoes, mash and season with salt and nutmeg. Fold in the zucchini cubes. Keep warm. Roast the sunflower seeds in a pan without fat until golden brown and remove.
Heat the remaining fat in the pan and fry the onion rings in it. Fill the zucchini with mashed potatoes and arrange on plates with the tomato sauce. Sprinkle with sunflower seeds and onions and serve garnished with parsley and thyme.