For the ranchero sauce: On a gas range or under a broiler, roast the green, red and poblano peppers until well charred. Let cool. Stem and seed the charred peppers. Coarsely chop and set aside.
In a spice grinder or with a mortar and pestle, finely grind the whole guajillo and pasilla chiles and set aside.
In a large saucepan, warm the oil over medium heat. Add the onions and cook, covered, 3 to 5 minutes. Add the guajillo and pasilla chiles, cumin, chili powder, oregano, thyme, black pepper, salt and garlic. Stir and cook until the spices become fragrant and are fully coated in oil, 3 to 5 minutes.
Add the tomatoes, green chiles and sugar. Stir and continue to cook until the sauce starts to simmer. Reduce the heat to medium-low and continue to cook until the vegetables soften and the flavors come together, 30 to 40 minutes. Taste and add additional salt and pepper as needed.
For the ranchero chicken: Preheat the oven to 325 degrees F. In a small Dutch oven or medium sized roasting pan combine the chicken, stock, salt, chili powder, cumin, garlic powder and lime juice with 2 cups of the ranchero sauce. (Reserve remaining ranchero sauce for another use.) Stir and top with the lid or seal well with aluminum foil and braise in the oven, 2 1/2 hours. Remove from the oven and let cool enough to handle the chicken.
Shred the chicken with two forks or your hands. Strain the braising liquid and add the strained solids back in with the shredded chicken. Reserve enough liquid to moisten the mixture. Return to the pan and keep warm in a low oven until ready to serve.
For the tostada shells: Meanwhile, line a baking sheet with paper towels. In a small frying pan, heat 1/2-inch oil until hot. Add 1 tortilla and cook, flipping once a minute, until crispy and golden brown, 3 to 5 minutes. Transfer to the paper towels and repeat with remaining tortillas.
Serve the chicken on the tostada shells and top with avocado and queso fresco.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.
Recipe courtesy of Lone Star Taco Bar, Allston, MA