Preheat the oven to 400°F. Pound the chicken breasts with a meat mallet or rolling pin to create an even thickness. Then cut the chicken into even strips about ½ inch thick. Place the chicken on a large baking sheet. You can line the baking sheet with aluminum foil first for easy clean up.
Wash and slice the bell peppers into even strips. Slice the onion into half moon slices. Add the bell peppers and onion to the same sheet pan as the chicken.
In a small bowl mix the cumin, chili powder, adobo seasoning, smoked paprika, salt, and black pepper together.
Drizzle the cooking oil on top of the chicken, peppers, and onion. Then sprinkle the seasoning mix and 1 tablespoon of chopped cilantro on top. Mix and toss the ingredients together with your hands until everything is well coated. Spread all of the chicken and veggies out into an even layer on the baking sheet.
Bake the sheet pan fajitas in the oven for 20-25 minutes or until the veggies are slightly charred and the chicken has reached 165°F.
While the fajitas are baking, warm the tortilla shells in a large ungreased skillet over medium-high heat for about 10-15 seconds per side. You can also warm them in the microwave on a microwave safe plate with a damp paper towel on top.
Once the fajitas are done, remove the sheet pan from the oven and finish by adding more fresh cilantro on top and fresh squeezed lime juice. Assemble the chicken fajitas with your favorite toppings and enjoy!