Combine potatoes and enough water to cover in a pot and bring to a boil. Reduce heat to simmer, cover and cook for 15 minutes. Drain and return to the pot or large saucepan. Mash the potatoes in the pot then add the butter and ½ teaspoon salt. Then gradually add the milk and mix until well blended. Set aside.
Preheat oven to 400 degrees F.
Heat 2 tablespoons of vegetable oil on a large cast iron pan over medium heat. Saute the onions and mushrooms for 2 minutes then add the carrots and bell peppers, cook for few more minutes until soft. Season with salt and pepper. Transfer to a plate. On the same pan add 2 tablespoons of oil and cook the chicken until done about 3 minutes. Add the thyme and season with salt and pepper. Stir in the ketchup and Worcestershire sauce. Mix the chicken broth and flour together then add to the pan. Stir until sauce thickened.
Top the chicken mixture with mashed potatoes. Place the pan on a large rimmed baking pan to catch any overflow. Bake for 25 minutes until lightly browned. Serve
Recipe by The Gluten Free at https://www.theglutenfree.com/gluten-free-chicken-shepherds-pie/