In a large pan over medium heat, melt 2 tablespoons of butter.
Sauté the onion and garlic until the onion is slightly translucent, about 5 minutes.
Add the green, red, and yellow peppers, and continue to cook until everything is caramelized, about 10 minutes more.
Remove the veggies from the heat, and set them aside in a medium bowl.
Stir in the taco seasoning, and season with salt and pepper.
Using a sharp knife, slice a pocket in the side of each chicken breast. Spoon about 2 tablespoons of the fajita vegetable mixture into each piece of chicken.
Season the outside of the chicken breasts with salt and pepper.
In the same pan over medium heat, heat the remaining 2 tablespoons of butter.
Cook the chicken for about 7 minutes on each side, or until golden and cooked all the way through.
Serve immediately.