Peel the potatoes and cut into 1 cm cubes. Peel and dice the onion as well. Fry both in the melted butter. Add caraway seeds and broth and simmer for approx. 15-20 minutes. Skin, core and puree the tomatoes and stir into the soup. Season everything with salt and pepper. Wash the basil, chop it slightly and add it to the soup. Serve hot.