Preheat oven to 350 degrees. I boil my chicken in water with salt and bay leaves for about 30 minutes on the stove top. I find the chicken is easier to shred that way. You can cook it however you like though. Peel and chop potatoes and boil in salted water for 20-25 minutes. In the meantime, add onions, garlic, and butter to a small sauce pan. Simmer for about 5 minutes then add flour to make a roux. Set aside. Heat soup, milk, and salt and pepper in a separate pan. Drain potatoes, place back in pot and mash with butter and half and half. Salt and pepper to taste. Mix chicken, peas and carrots, soup mix, and the roux together and pour into a casserole dish sprayed with non stick cooking spray. Top with potatoes. Place in the oven and bake for 45 minutes.
***Optional: Add cheese or bread crumbs to the top.