Heat 2-3 tablespoons of butter in a large cast iron or saucepan over medium heat.
Add in the mushrooms and cook until mushrooms are soft, about 4-5 minutes. Remove and set aside.
Melt an added 2-3 tablespoons butter, followed by all-purpose flour. Stir for about a minute.
Then add onions, thyme, and garlic. Cook and stir for about 4-5 minutes, until tender.
Gradually pour in half and half and the chicken broth. Stir to fully combine.
Bring to a boil; cook and stir for about 2 minutes or until thickened and bubbles appear.
Stir in celery and carrots, continue cooking for 3-5 minutes.
Return the sautéed mushrooms back to the pot together with cooked chicken, frozen peas, chopped pimento, Creole seasoning, and parsley. Season with salt and pepper.
Let it cook for about 2-3 minutes or until the chicken has been heated through.
Adjust with more broth, salt, pepper, and seasonings to taste.