Put the onions, celery and chopped peppers with the oil in your largest flameproof casserole dish or heavy-based saucepan, and fry gently over a low heat until soft but not coloured.
Drain the jar of peppers over a bowl to catch the juices. Put a quarter of the peppers into a food processor with the chipotle chilli, ancho chilli, vinegar, cocoa, dried spices and herbs. Whizz to a purée, then stir into the softened veg and cook for a few mins.
Add the tomatoes and refried beans with 1 can water and the reserved pepper juice. Simmer for 1 hr until thickened, smoky and the tomato chunks have broken down to a smoother sauce.
Add the black beans and mixed beans along with the remaining roasted peppers, cut into bitesize pieces and take off of the heat.
For the cornbread dumplings, combine the polenta, flour, bicarb and salt in a mixing bowl and stir in 40g of the cheese, the coriander and some freshly ground black pepper. When the chilli is ready stir the yogurt and sweetcorn into the dry ingredients to make a sticky dough - if it seems to dry add a little milk. Drop rough spoonfuls on top of the chilli, scatter with the rest of the cheese and transfer to the oven. Bake uncovered for 20 minutes, or until the topping is cooked through and crunchy.