Preheat oven to 400℉. Spray an 8x8 casserole dish with non-stick spray.
Cut potatoes into chunks. If leaving the potatoes unpeeled, cut into small chunks. Place in a large pot, and cover with water. Bring to a boil and simmer until soft, about 20-25 minutes, depending upon the size of the potatoes. Drain thoroughly.
Use a potato masher to smash the potatoes into small chunks.
Add sour cream and half & half to the potatoes. Using the potato masher or a hand mixer, mash or whip to desired consistency. Season to taste with salt and pepper.
Meanwhile, add olive oil to a large pan and add onions, carrot, and celery. Stir and sauté then mixture for 5 minutes as the vegetables start to soften. Add the ground turkey to the pan. Break up the meat into small pieces as it cooks. Add the garlic and cook for one minute.
Stir in flour and cook another minute. Add frozen peas, chicken stock, ketchup, Worcestershire sauce and herbs. Season with salt and pepper and simmer 5 minutes as the mixture thickens.
Spread the ground turkey mixture on the bottom of the prepared casserole dish. Top with the mashed potato mixture, leaving some peaks and ridges on the top.
Bake for 25 minutes until the turkey mixture is bubbling and the potatoes are golden.