Peel and finely chop the onion. Halve the pepper, scoop out any seeds and white pith, then finely chop it to match the onion. Trim the dry ends from the celery stick and finely slice it.
Warm a large pan over a medium heat for 2 mins, then add the chopped veg with ½ tbsp oil. Season with a little salt and pepper. Cook for 5 mins, stirring occasionally, till the veg have softened.
While the veg fry, peel and grate or crush the garlic. Halve the chilli. For less heat, flick out the seeds and white pith, then finely chop the chilli. Drain the beans and rinse them under cold water.
Stir the garlic and chilli into the veg. Add 1 tsp oregano and 1½ tsp Cajun seasoning. Stir in the black beans and the bulgar wheat. Tip in the tin of chopped tomatoes. Fill the can with water (around 400ml) and add it to the pan. Pop a lid on the pan, bring to the boil, then turn the heat down and simmer for 20-25 mins, stirring now and then, till the chilli has thickened and the bulgar is tender.
While the chilli simmers, heat your oven to 180°C/Fan 160°C/Gas 4. Slice the tortillas into around 8 triangles each. Spread them out on a baking tray. Bake in the oven for 5-10 mins till golden and crisp.
Meanwhile, coarsely grate the cheddar. Finely chop the chives.
Taste the chilli and add a pinch more salt or pepper if you think it needs it. Spoon the chilli into 2 warm bowls. Top with the grated cheddar, chopped chives, spoonfuls of the yogurt and a pinch more of the Cajun seasoning. Serve with the tortilla chips for scooping.