Finely chop the onion and bell pepper. Pass the garlic through a press to make about 1 tablespoon of garlic. Cut the avocado into cubes, if using. Heat olive oil in a large nonstick skillet over medium heat. Add onion, pepper and ½ teaspoon of salt. Fry until the onions begin to brown, about 5 minutes. Add garlic, chili powder, cumin, cayenne pepper and fry until aromatic, about 30 seconds.
Add pasta, chopped tomatoes with juice, water and beans. Cover the pan with a lid and bring to a boil. Reduce heat to medium and simmer, stirring occasionally, until the paste is soft, about 20 minutes.
In a small bowl, combine about half of the boiling mixture in a skillet with Greek yogurt. Pour the mixture into a paste. Cover and simmer over low heat until warmed up for 2-3 minutes. If the sauce is too runny, simmer for a few more minutes. Sprinkle coriander over the dish and garnish with avocado slices if desired. Serve immediately.