1) To make the tortillas: Pour the oil for frying into a large heavy-bottomed pan to a depth of about 5cm. Place over medium heat and heat to 190C/Gas Mark 5. Add the tortillas, one at a time, and fry until golden brown and crispy, about 1 1/2 minutes. Using tongs, transfer to a paper towel-lined pan and sprinkle with salt. Set aside.
2) To make the chicken: In a small bowl, mix together the chicken, lime juice, oil, coriander, and salt and season with pepper to taste. Cover with clingfilm and set aside.
3) To assemble the tostadas: Preheat the grill. Spread the beans evenly over one side of each tortilla and sprinkle with some of the cheese. Transfer to a baking sheet and grill until the cheese is lightly browned and melted, about 30 seconds. Divide the tortillas among 6 plates.
4) Evenly mound the chicken on each tortilla and top with the lettuce. Top each tostada with avocado, salsa, and a dollop of sour cream. Sprinkle with the spring onion and garnish the tostada with the coriander leaves. Serve immediately.
Cook's Note: The tortillas must be fried until very crispy, otherwise they may become rubbery, making them difficult to cut with a knife and fork.
5) For the refried beans: Heat the oil in a large pan over medium-high heat. Add the onion, coriander, and cumin and cook, stirring, until lightly browned, about 2 minutes. Add the garlic and cook, until lightly browned, about 1 minute more. Add the beans and cook, stirring frequently, until thick and amber brown in color, about 4 minutes. Stir in the salt and season with pepper to taste.
6) For the salsa cruda: In a small bowl, mix together the tomatoes, onion, jalapeno, coriander, salt, and season with pepper to taste. Cover with clingfilm and set aside.