Preheat oven to 375°. In a shallow dish, combine chicken breast with olive oil, salt, cayenne, chili powder, paprika, cumin and garlic powder until covered evenly. Bake for 30 minutes or until chicken reaches an internal temperature of 165°.
Meanwhile, blend ranch dressing with poblano pepper until smooth. This can be made ahead and stored in the fridge in air tight container for up to two weeks.
Once chicken is done, shred it with two forks in the dish. Allow chicken to soak up juices. Set aside.
In a high sided skillet, heat oil to 350° or until it bubbles when a piece of tortilla is placed in it.
Working in batches of four, gently fry tortillas, about 10 seconds on each side, until puffy and slightly golden brown. Set on a paper towel lined plate to rest. Tortillas should be pliable and have a chewy texture.
Once tortillas are cool enough to handle but still very warm, fill with about 1 tablespoon of shredded chicken and about 1/2 tablespoon each of onion and cheddar. Fold in half and place on baking sheet lined with parchment paper or cooking mat.
Finish baking tacos for 5 to 10 minutes at 375° or until cheese is melted and bubbly and taco shells are slightly crisp on edges.
Garnish with cilantro and poblano ranch and eat hot!