Heat the oil in a large pan and cook the onion over a medium heat for 10 mins to soften. Stir in the chipotle paste, passata, stock and all the beans. Season, bring to the boil, then gently simmer for 5 mins.
Meanwhile, in a bowl crumble the feta and mix with the garlic. Divide between the tortillas, spreading over one half of each, then sprinkle over a little pepper. Fold the uncovered side over and press down. Heat a dry frying pan and cook the tortillas on both sides for a couple of mins until the feta has melted and the tortillas are crisp.
Divide the soup between bowls, scatter with coriander and serve with the tortillas.