- MAKE THE MEAT FILLING:
- Heat the oil in a large skillet over medium-high heat. Add the onions and cook for 5 minutes, stirring occasionally.
- Add the ground beef or lamb, parsley, rosemary, thyme, salt, and pepper. Cook for 6-8 minutes, until browned, stirring occasionally.
- Stir in Worcestershire sauce and garlic. Cook for 1 minute.
- Add flour and tomato paste, stirring until well incorporated.
- Pour in the broth, add frozen peas, carrots, and corn. Bring to a boil, then reduce heat and simmer for 5 minutes. Set aside.
- MAKE THE POTATO TOPPING:
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- Place the potatoes in a large pot, cover with water, and bring to a boil. Reduce heat and simmer until fork-tender, 10-15 minutes.
- Drain the potatoes, return them to the pot, and let them sit for 1 minute to evaporate any remaining liquid.
- Add butter, half & half, garlic powder, salt, and pepper. Mash and stir until well combined.
- Mix in parmesan cheese.
- ASSEMBLE THE CASSEROLE:
- Preheat the oven to 400°F.
- Pour the meat mixture into a 9×9 (or 7×11) inch baking dish and spread it into an even layer.
- Spoon the mashed potatoes on top of the meat and spread into an even layer.
- Place the baking dish on a rimmed baking sheet to catch any spills. Bake uncovered for 25-30 minutes.
- Cool for 15 minutes before serving.