Cut the squid tubes in half horizontally and cut into pieces. Peel onion and garlic and chop finely. Heat the oil in a pot. Fry the squids for about 3 minutes, turning them over.
Add onion and garlic and fry for 1-2 minutes, season with salt and pepper. Add tomatoes and vegetable stock. Chop the tomatoes slightly with a spatula. Cover and simmer for about 45 minutes.
Approximately 10 minutes before the end of the cooking time, add the capers and cook at the same time. Season to taste with salt, pepper and sugar, allow to cool slightly and serve.