How to Make It
In a large bowl, combine 2 tablespoons of the lime juice with 1 teaspoon of salt, stirring to dissolve the salt. Add the chicken thighs and turn to coat. Refrigerate for 2 hours.
In a food processor, combine the tomatillos, garlic, serranos, onion and cilantro and process to a coarse puree. Transfer the salsa to a bowl and stir in the avocado and the remaining 2 tablespoons of lime juice. Season with salt.
Pat the chicken dry with paper towels. In a shallow bowl, combine the flour with the pepper, cayenne and 1/2 teaspoon of salt.
In a large skillet, heat 1/3 inch of oil until shimmering. Dredge the chicken in the flour, shaking off any excess. Fry the chicken over moderately high heat until well-browned and crisp, about 3 1/2 minutes per side. Transfer to paper towels to drain.
Cut the chicken into thin strips and serve with the tomatillo salsa and warm corn tortillas.