Roughly chop the onion and garlic. Heat half of the oil in a frying pan. Sauté the onion and garlic. Extinguish with the tomatoes and cook on low heat for fifteen minutes.
Cut the chilies lengthwise into thin strips. In a frying pan, heat the remaining oil and saute the chili strips very well. Remove from heat and cool.
Add the soup to the tomatoes and season with salt and freshly ground pepper. Continue to cook on low heat for ten minutes.
Meanwhile, break the tacos into large pieces. Cut the avocado in half and remove the stone. Peel off the skin and dice the avocado.
Divide soup evenly into plates. Sprinkle with feta cubes, chile strips, avocado cubes and taco pieces. Garnish with sour half cream and cilantro leaves.
Add half a lime for each guest to squeeze equally, so that the soup can still be individually flavored.