Heat the vegetable oil in a large shallow frying pan over a medium-high heat until it shimmers. Fry the tortillas, one at a time, until golden, turning once or twice and pressing with tongs to immerse. This should only take about 45 seconds per tortilla. Drain on kitchen paper and transfer to two serving plates.
Discard the oil and, in the same frying pan over a high heat, fry the beef along with the chilli powder for 3-5 minutes, or until cooked. Tip in the chilli beans and stir until heated through.
To serve, divide the chilli beef and bean mixture between the crispy tortillas, then top with the shredded lettuce, cherry tomatoes and avocado slices. Garnish with the natural yogurt and serve immediately.