Prepare mashed potatoes according to the instructions on the box and then set the mashed potatoes aside.
Preheat your oven to 375°F (190°C).
In a large skillet or over medium-high heat, cook the lean ground turkey until it's no longer pink, breaking it apart with a wooden spoon or meat masher as it cooks. Season the browning meat with salt, black pepper, onion powder and garlic powder.
In the same skillet, add the frozen mixed vegetables. Stir and cook for 5 minutes until the veggies are heated through.
Pour the marinara sauce, chicken broth, Worcestershire sauce and turkey gravy over the meat mixture. Stir the ingredients well to combine. Let it simmer for 5-7 minutes until the sauce thickens slightly. Taste and adjust seasoning with salt and pepper if needed.
Transfer the turkey and vegetable mixture to a lightly greased 8x8-inch baking dish and spread it out in an even layer. Spoon the prepared mashed potatoes over the top of the pie filling, spreading them evenly with a wet rubber spatula to cover the entire surface.
Place the baking dish in the preheated oven and bake for 25-30 minutes, or until the mashed potatoes start to turn golden brown.
Remove the casserole from the oven and let it cool for a few minutes before serving. Serve hot and enjoy your easy turkey shepherd’s pie!