1) Season the chicken thighs generously with salt and pepper. Heat the vegetable oil in a large saucepan over high heat. Brown the chicken well on both sides. Remove chicken and set aside.
2) Reduce heat to medium and saute onion, garlic, oregano and cumin for a few minutes. Stir in tomatoes, lime juice, sugar and salt. Add chicken back to the sauce, bring to a simmer, cover and cook for 45 minutes.
3) Remove chicken from cooking liquid and let rest until cool enough to handle. Pull the meat from the bone and toss with the cooking liquid. Discard the bones and skin.
4) Rub a cast iron skillet lightly with vegetable oil and heat over medium flame.
5) Sprinkle cheese all over the tortilla. Top with pulled chicken, serrano chillies and another tortilla. Toast on each side in the skillet until warmed through and cheese has completely melted. Repeat with the remaining ingredients.