While potatoes are cooking, preheat the oven to 425℉ and heat oil in a cast iron skillet or large pot over medium heat.
Once hot, add onions and sauté for about 2 minutes or until onions start to turn translucent.
Crumble the ground turkey into the cast iron skillet and cook, breaking it up into pieces with your spoon. Cook until the turkey is cooked through and crumbly, about 5-7 minutes.
Add garlic powder, salt, Worcestershire sauce, poultry seasoning, and soy sauce to the skillet. Sauté for 2 minutes, stirring often.
Add broth and frozen vegetables to the skillet. Stir to combine.
In a small bowl or large measuring cup, thoroughly mix cornstarch and milk together. Make sure there are no clumps at the bottom. Add the cornstarch mixture to the skillet and bring it to a simmer. Simmer for 5 minutes or until sauce is thicker and the vegetables are warmed through.
Either use the cast iron skillet as your baking dish or transfer the filling into a 9" x 13" greased baking dish. Spread the filling evenly in the baking dish and then top with the mashed sweet potatoes. Spread the potatoes in an even layer, mashing them down gently with a fork to cover the filling. Bake 15 to 20 minutes until the potatoes are lightly browned on top. For a crisper potato topping, broil for 1 to 2 minutes. Garnish fresh herbs and enjoy warm.
Let leftovers cool completely before storing in the fridge for 3-4 days.