I whisked lime juice, olive oil, garlic, cumin, chili powder, cilantro, salt, and pepper in a bowl. Then I added the chicken thighs (trimmed of any excess fat), pressed the marinade over them in a zip-top bag, and refrigerated for at least 1 hour (or up to 6 hours) to let the flavors infuse.
I preheated my grill to medium-high and brushed it lightly with oil. I placed the thighs on the hot grill and cooked about 5 minutes per side, until nicely charred edges and internal temperature reached 165 °F. Then I removed them and let them rest briefly.
Once rested, I diced the chicken into bite-size pieces. I warmed the corn tortillas (I sometimes double them to prevent tearing), laid down the chicken, and topped with sliced avocado, chopped onion, extra cilantro, and a squeeze of lime.