In a wide 4 quart/4L pan, heat the butter, or oil, over medium high heat. Add the sliced chicken thighs, and cook until lightly browned, 6 - 8 minutes.
Add the diced onion and diced bell pepper to the pan. Sprinkle the taco seasoning over everything and stir to combine. Lower heat to medium, cook until onion starts to become translucent, 6 - 8 minutes. Add garlic and stir to combine.
Add taco sauce, diced tomatoes, chicken stock, and pasta to the pan. Stir to combine, then bring to a simmer, cover with a tight-fitting lid and lower heat to medium low if it starts to boil - you want it to simmer, not boil. Cook for 10 - 12 minutes, or until chicken and pasta are cooked through. (Check the cooking time on your box of pasta.)
Remove from heat. Add half and half, and 1 1/2 cups of shredded cheese. Stir for a couple of minutes until the cheese is melted and everything is combined.
Garnish with remaining shredded cheese and chopped cilantro on top. Serve.