Prepare the Meat Filling

1. Preheat your oven to 400°F (200°C).
2. In a large skillet, heat the olive oil over medium heat.
3. Add the diced onion and cook until translucent, about 5 minutes.
4. Add the minced garlic and cook for another minute.
5. Add the ground beef or lamb to the skillet, breaking it apart with a wooden spoon.
6. Cook until browned and no longer pink.
7. Stir in the mixed vegetables and cook for 5 minutes.
8. Add the tomato paste, Worcestershire sauce, beef broth, dried thyme, and dried rosemary.
9. Stir to combine and simmer the mixture for 10 minutes until the sauce has thickened.
10. Season with salt and pepper to taste.
11. Transfer the meat filling to a baking dish and spread it out evenly.

Prepare the Mashed Potato Topping

1. While the meat filling is simmering, place the diced potatoes in a large pot and cover with water.
2. Bring to a boil and cook until the potatoes are tender, about 15 minutes.
3. Drain the potatoes and return them to the pot.
4. Add the milk and butter to the potatoes and mash until smooth and creamy.
5. Season with salt and pepper to taste.

Assemble and Bake the Shepherd’s Pie

1. Spread the mashed potatoes evenly over the meat filling in the baking dish.
2. If desired, sprinkle the shredded cheddar cheese on top of the mashed potatoes.
3. Bake in the preheated oven for 20-25 minutes, or until the top is golden brown and the filling is bubbling.
4. Allow the shepherd’s pie to cool for a few minutes before serving.