First make the bean chilli. Heat the oil over a medium heat in a large saucepan or a flameproof casserole dish and add the onion, pepper and celery. Cook for approx. 5 minutes, until soft and translucent, then add the garlic and fresh chillies. Continue to cook for another couple of minutes.
Stir in the chilli flakes, chipotle paste, cumin, oregano, tomatoes and beans. Season with salt and freshly ground pepper and add 100ml of water.
Bring to the boil, then turn the heat down to a simmer and leave to cook, uncovered, for 15 to 20 minutes until the sauce has reduced down. Stir regularly to make sure it isn’t catching on the bottom. The consistency should be quite thick, if not dry.
Meanwhile, cook the rice - put it in a pan with the lime zest, coriander stems and 150ml of water. Season with salt, bring to the boil and cover. Turn down the heat and simmer for about 15 minutes until all the water has been absorbed. Take the pan off the heat and remove the lid. Put a tea towel over the top of the pan and place the lid on top. Leave the rice to steam in its own heat for a further 10 minutes.
To assemble, heat each tortilla on a dry frying pan for just a few seconds on each side to make them easier to fold. Spoon the chilli into the centre of the tortillas in a roughly rectangular shape and add the rice and as many of the optional toppings as you want. Fold over the sides, then fold over one of the remaining edges and roll. Wrap the burritos in foil to make them easier to hold while eating.