Make the filling
1. Preheat oven to 180 degrees C.
2. Heat olive oil in a large oven-safe pot over medium-high heat. Add the onions and cook for about 5 minutes, stirring occasionally.
3. Add the mince and break it up with a wooden spoon or spatula. Sprinkle in the dried herbs and a pinch of pepper. Cook for 6-8 minutes, or until the mince is browned.
4. Stir in the Worcestershire sauce and garlic and cook for another minute.
5. Add the flour and tomato paste. Mix well so everything is combined and the tomato paste is evenly stirred through.
6. Pour in the stock and add the lentils and frozen vegetables. Bring to a boil, then reduce to a simmer for 5 minutes.
7. Turn off the heat and set the meat mixture aside.
Make the potatoes
8. Add potatoes to a large pot and cover with water. Bring to a boil and cook until the potatoes are soft, about 10-15 minutes.
9. Drain the water, then return the potatoes to the pot.
10. Add milk, garlic powder, and pepper. Mash until smooth and combined.
Assemble
11. Spoon the mashed potato over the top of the meat mixture in the oven-safe dish, and gently spread it out to cover the filling.
12. Top with cheese.
13. Bake uncovered for 25-30 minutes, until the top is lightly golden.
14. Let it cool for about 15 minutes before serving so it sets nicely and is easier to scoop.