Peel the vegetable onions and cook in boiling salted water for about 15 minutes. Peel, wash and halve the potatoes and cook in boiling salted water for about 20 minutes. Cut the sausage into slices. Heat 1 tablespoon of oil in a frying pan and fry the sausage in it over medium heat for about 5 minutes, turning
Remove the onions from the water and hollow them out. Chop the inside of the onion. Heat 2 tablespoons of oil and fry the chopped onion for about 5 minutes, turning. Wash the thyme, shake dry, remove leaves from the stems except for a little garnish. Add tomatoes and stock. Season with half the thyme, salt and pepper
Heat milk and butter in a pot. Grate cheese. In the meantime, drain the potatoes, add the milk mixture and mash them with a potato masher to make puree. Season to taste with salt, nutmeg and thyme. Fold in chorizo
Season hollowed onions inside and outside with salt. Spread mashed potatoes into the onions and sprinkle with cheese. Put the tomato sauce in an ovenproof dish. Place the onions on top. Bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for approx. 45 minutes