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 translucent, about 5 minutes.
Stir in the minced garlic and cook for an additional 1 minute until fragrant.
Add the ground beef to the skillet, breaking it apart with a spatula. Cook until browned, about 7-10 minutes. Drain excess fat if necessary.
Stir in the mixed vegetables, tomato paste, Worcestershire sauce, thyme, rosemary, salt, and pepper. Cook for another 5 minutes until the vegetables are tender.
Spread the beef and vegetable mixture evenly in a 9x13 inch baking dish.
Top the mixture with the prepared mashed potatoes, spreading them evenly over the beef layer. If desired, sprinkle shredded cheddar cheese on top.
Bake in the preheated oven for 25-30 minutes, or until the top is golden brown and the filling is bubbling.
Let it cool for 5-10 minutes before serving.