To make the tortillas: Pour the oil for frying into a large heavy-bottomed pot to a depth of about 2 inches. Place over medium heat and heat to 375 degrees F. 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.
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 plastic wrap and set aside.
To assemble the tostadas: Preheat the broiler. Spread the beans evenly over one side of each tortilla and sprinkle with some of the cheese. Transfer to a baking sheet and broil until the cheese is lightly browned and melted, about 30 seconds. Divide the tortillas among 6 plates.
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 scallion and garnish the tostada with the coriander leaves. Serve immediately.
Heat the oil in a large skillet 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.
In a small bowl, mix together the tomatoes, onion, jalapeno, coriander, salt, and season with pepper to taste. Cover with plastic wrap and set aside.
The tortillas must be fried until very crispy, otherwise they may become rubbery, making them difficult to cut with a knife and fork.
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