In a large skillet melt the chicken fat (or butter) on high heat. Add the carrots, celery, and onions and cover with a lid. Sweat the vegetables for 5 minutes, stirring occasionally. If using butter, you may need to add a little chicken stock to the pan for additional liquid to sweat.
Add in the poultry seasoning, mushrooms, and penne pasta. Mix well and add the chicken broth. Cook on medium heat with lid on for 20 minutes, until pasta is tender. Add in the frozen green peas during the last 5 minutes of cooking.
While pasta cooks, mix together the heavy cream and cornstarch in a small bowl. Set to the side.
Once the pasta is done cooking, turn off the heat and add in the chopped chicken. Mix in the heavy cream mixture and let stand for at least 5 minutes with the lid on. Season with salt and pepper to taste and enjoy!