Peel the onions and cut off the lid. Steam onions in a large pot with little water for 30-40 minutes. Bring 375 ml water and 1/2 teaspoon salt to the boil. Remove the pot from the heat, add cold milk and stir in the puree powder with a wooden spoon.
Wash the thyme, dab dry, put about 4 stems aside for garnishing, chop the rest finely. Wash, clean, quarter and seed the tomatoes. Cut the flesh into small cubes. Remove the onions and chill them cold.
Hollow out the onions with a ball cutter. Finely chop approx. 80 g onion interior. Heat the oil in a large pan. Fry the ground pork in it until crumbly. Add chopped onions and fry until translucent. Add the minced pork fat and half of the diced tomatoes to the puree and mix.
Fill the onions with the meat and potato mixture and put them into an ovenproof dish. Grill the onions under the grill of the oven for 5-10 minutes until golden brown. Meanwhile melt the fat, dust with flour and sauté for about 2 minutes.
Deglaze with cream and stock, bring to the boil. Melt cheese in it, add remaining tomato cubes and chopped thyme, boil down briefly to a creamy sauce and season with salt and pepper. Remove the onions from the oven and garnish with the rest of the thyme.
Serve with the sauce on plates.