Preheat griddle on low to medium until it reaches about 400°F.
Place the marinated chicken thighs on the heated griddle with the skin side (smoother side) facing down. Allow the chicken thighs to cook on the first side until golden brown, approximately 5-7 minutes depending on the griddle/outside temp. Then flip the chicken thighs to the second side and continue to cook until the fattest part of each thigh reaches an internal temperature of 165°F. As each individual piece of chicken reaches the correct temperature, remove from the griddle and transfer to a cutting board. *Note - The chicken will cook at different times, so it's important to temp each piece before taking it off the griddle.
When all of the chicken is done cooking, and without cleaning or scraping the griddle surface, add the sliced onions and bell peppers to the griddle. Add salt and pepper to the vegetables and allow to cook untouched for about 3 minutes before starting to saute. As you start to mix the vegetables, use your spatula to scrape the excess seasoning from the chicken off the griddle surface to flavor your vegetables.
When onions and peppers are almost done cooking, add the sliced tomatoes and saute the vegetables together.
While the vegetables finish cooking, cut the cooked chicken thighs into slices. Taste the cut cooked chicken for seasoning and add more seasoning if needed.
Add the cut chicken to the griddle, and add fresh squeezed lime juice to the chicken. Mix the chicken and the vegetables together and saute for 2-3 minutes until everything is combined.
Remove the chicken fajitas from the griddle and serve warm. *If you are cooking a large batch for a crowd, you can transfer the chicken fajitas to a large pan and cover with aluminum foil. Place the chicken in the oven covered on the warm setting (about 170°F) until ready to serve.
Serve chicken fajitas with warm tortillas and your choice of fajitas toppings.