Heat a dry, heavy-based saucepan over a medium heat and char the tomatoes and garlic. Remove from the heat and peel when cool enough to handle.
Place the lard in a saucepan over a medium-low heat and gently fry the onions until softened but not coloured. Add the garlic and cook for a further 4–5 minutes. Add the chipotles, tomatoes, oregano and stock. Cook for 10–15 minutes, until everything is soft and pulpy. Blend until smooth. Season with salt and pepper, add the shredded chicken, if using, cover and keep warm over a low heat.
Pour the oil into a frying pan to a depth of about 2.5cm/1in. Heat the oil until a small cube of bread sizzles and turns golden-brown when dropped into it. (CAUTION: hot oil can be dangerous. Do not leave unattended.). Fry the tortilla strips in batches until golden and crisp. Remove the strips with a slotted spoon and drain on kitchen paper.
Toss the avocado cubes with the lime juice and set aside.
Divide the soup between four bowls and top with strips of tortilla, a sprinkling of chipotle flakes, diced avocado, cheese, soured cream and coriander.