Preheat oven to 350°F. In a small bowl, stir together mayonnaise, garlic, lemon or lime juice, and chipotle chili powder. Add salt and pepper to taste. Set aside.
Sprinkle taco seasoning over chicken thighs until lightly coated on all sides. In a large skillet, over medium-high heat, heat vegetable oil. Once hot, add chicken thighs and cook for 3 to 5 minutes per side, until an instant-read thermometer reads 165°F. If the outside cooks too quickly, turn down the heat.
While meat cooks, in another large skillet, heat vegetable oil over medium heat and add onion and bell peppers. Sprinkle with taco seasoning and toss to coat. Cook, stirring occasionally, for 4 to 8 minutes, until edges are lightly browned and vegetables are tender-crisp. Add salt and pepper to taste.
For softer vegetables, turn heat down to medium once browned and cook until desired doneness.
Slice chicken thighs into strips. Spread aioli over both sides of each roll and layer chicken, fajita vegetables, and cheese on the bottom half of each roll.
Place rolls in the oven and bake for 5 to 6 minutes until cheese is melted and rolls are toasty. Serve and enjoy!
