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Heat the oil in a non-stick frying pan and fry the onion and garlic for 8 mins. Add the pepper and spices to taste and fry for 1 min more.
Tip in the tomatoes and a can of water, stir in the oregano, bouillon and quinoa, bring to the boil, then cover and simmer for 20 mins.
Stir in the black beans and cook, uncovered, for 5 mins more. Add most of the coriander, then serve topped with the yogurt (if using), the remaining coriander and the avocado slices.