Clean and chop up the onion into long strips. Slice the peppers into long thin strips. Clean and slice the chicken breasts into long thin strips and place in a bowl.
Add a tbsp of the fajita mix to the chicken breast and mix well.
In a large skillet, heat 1 tbsp of the butter and add the chicken to it. Cook the chicken for 5 to 10 minutes until it's fully cooked and no longer pink, and it's slightly charred, but not burned.
Remove chicken from skillet, and wipe the skillet clean.
Add the other 2 tbsp of butter to the skillet and heat it up. Add the onion and cook for a couple minutes until it's slightly translucent. Add the peppers and the fajita mix and stir well. Cook for a couple more minutes so the peppers cook slightly. Add the chicken to the skillet, mix with peppers and onions and cook for a minute.
Preheat oven to 400 F degrees. Lightly butter a baking sheet with the melted butter, I used a 9x13 inch baking dish.
Lay a tortilla flat and top some chicken fajita mixture then top with some cheddar cheese, about a tbsp or two. Tightly roll up the tortilla and place it in the baking dish. Brush with melted butter. Repeat with remaining tortillas.
Bake for about 15 minutes or until tortillas are nicely golden brown.
Serve with salsa and/or sour cream.