Sprinkle chicken thighs with taco/fajita seasoning until coated on all sides.
On the stove, heat 1 tablespoon of oil in a large skillet over medium-high heat. Once hot, add chicken thighs and cook for 3 to 5 minutes per side, until the internal temperature reaches 165°F. If at any time the outside of the chicken begins to cook too quickly and burn, turn the heat down.
While your meat cooks, in another large skillet, heat remaining 1 tablespoon of oil over medium-high heat. Once oil is hot, add bell pepper and onion slices. Sprinkle 1/2 teaspoon of taco seasoning or 1/4 teaspoon each of cumin and chili powder over the top. Cook, stirring occasionally, for 4 to 8 minutes, until edges are lightly browned and vegetables are tender-crisp. Salt and pepper to taste.
For softer vegetables, turn heat down to medium once the vegetables begin to brown so they don’t burn, and continue to cook until vegetables reach your desired level of doneness. For very soft vegetables, add 1/4 cup of water to the pan at this point, and cook until veggies are soft and water has evaporated, adding more water as necessary.
Slice cooked chicken thighs and serve over warmed tortillas with the vegetables, a squeeze of lime juice, and your favorite fajita toppings.