Preheat oven to 375 degrees. Dice the chicken from a chicken you have cooked or a rotisserie chicken. I cooked mine in the instant pot. I placed the chicken in the pot, adding water up about 3/4 to the top of the chicken. I added 2 bay leaves, a stalk of celery, 2 carrots, 1/2 onion and a little salt. I set the Instant Pot for 30 minutes and allowed it to release on it's on. I then remove the chicken and strain the broth, discarding the vegetables. When cool, you can dice or tear the chicken into pieces. You will probably have more chicken than you need left, so use it for another recipe. If you have a rotisserie chicken it will work, you'll just need some chicken broth in your pantry. Cook your spaghetti according to packaged instructions.
While your pasta is cooking heat the butter and oil in a large pot. Add the mushrooms and cook until the water has evaporated from the mushrooms and they are starting to brown. Don't salt your mushrooms until they begin to brown because the salt will just release the water. When they start to brown add the salt and pepper. Remove from the pan and set aside.
Add your onions and peppers and cook until the onions are translucent. When tender add the mushrooms back in. Add all of your seasonings, stir for a minute or so. Add a tablespoon of butter or oil, then add the flour. Cook the flour and vegetable mixture for a minute or so.
While stirring add your heavy cream and chicken stock and stir to make your sauce. Add the drained pimento. Add the velveeta or american cheese along with 1/2 of the cheddar cheese, and cook on low to medium until the cheese melts. Add your chicken and combine.
If your pan is large enough to add the pasta do so, otherwise, add the spaghetti to a baking dish large enough for you to toss all the ingredients.
Cover and cook for 30 minutes until bubbly, then remove the foil and add your cheese. Cook an additional 15 minutes or until your cheese has melted and the casserole is bubbly. Enjoy!
