Preheat oven to 350°. In a large stockpot, heat 1 tablespoon oil and add the onion. Cook until onion is translucent, about 5 minutes.

Add cabbage, carrots and green pepper (and more oil if necessary) and cook until cabbage starts to soften, about 3 minutes more. Stir in oregano, cumin, chicken broth and tomatoes with juice. Bring to a boil; then reduce heat and simmer for 20 minutes. To avoid turning the soup too dark, add the black beans during the last few minute sof simmering time.

While the soup is simmering, place the corn tortilla strips on a baking sheet. Toss with remaining 2 teaspoons oil and sprinkle with salt. Bake for 7-10 minutes until golden brown.

Serve soup garnished with avocado slices and tortilla strips.