Peel and finely dice the onion and garlic. Heat the oil, fry the onion and garlic for about 4 minutes until transparent. Add tomato paste after about 2 minutes. Dust with flour. Add the diced tomatoes while stirring.
Bring to the boil and simmer for about 10 minutes, stirring occasionally. Season to taste with salt, tabasco, sugar and vinegar. Wash the basil, shake dry, pluck leaves from the stalks and cut into strips, except for a little to garnish.
Mix the sour cream and milk. Serve the soup with sour cream and basil strips. Garnish with basil.