In a large shallow bowl, whisk together 2 tablespoons fresh lime juice, 2 tablespoons oil, 1 large minced garlic clove, 1 1/2 teaspoons ground cumin, 1 teaspoon salt, 1/2–1 teaspoon ground chili, and 1 tablespoon chopped cilantro (optional).
Add 1 1/2 pounds skinless, boneless chicken thighs to the marinade and toss to coat; if possible, marinate for 30 minutes for more flavor.
Heat a 12-inch (30 cm) skillet over medium-high until very hot. Add a drizzle of oil to lightly coat the pan.
Sear the chicken thighs, turning occasionally, until golden, lightly charred, and cooked through (about 6–8 minutes per side depending on thickness). Transfer to a plate, loosely tent with foil, and let rest briefly.
In the same skillet, add the sliced peppers and thinly sliced red onion and cook, stirring occasionally, until the onion is soft and the peppers are slightly charred and tender; add a little oil if the pan is dry. Season with salt and pepper to taste.
Slice the rested chicken into strips and return to the skillet with the peppers and onions for a quick toss to combine and warm through.
Serve the chicken and vegetables with warmed tortillas, chopped cilantro, and sliced avocado.
