Preheat your oven to 400°F (200°C).
In a large skillet, heat the olive oil over medium heat. Add the diced onion and cook until softened, about 5 minutes.
Add the minced garlic and cook for an additional minute until fragrant.
Stir in the ground turkey, breaking it up with a spoon, and cook until browned, about 7-10 minutes.
Add the frozen mixed vegetables, tomato paste, Worcestershire sauce, thyme, rosemary, salt, and pepper. Stir well and cook for another 5 minutes until the vegetables are heated through.
Transfer the turkey mixture to a 9x13 inch baking dish, spreading it evenly.
Spoon the mashed potatoes over the turkey mixture, spreading them out to cover completely. If desired, sprinkle the shredded cheddar cheese on top.
Bake in the preheated oven for 25-30 minutes, or until the top is golden and the edges are bubbling.
Let it cool for a few minutes before serving.