Preheat oven to 400°F.
In a large skillet, heat a bit of oil over medium heat. Add the onion and garlic, and cook until soft.
Add the ground lamb, breaking it up with a spoon. Cook until browned.
Stir in the tomato paste, Worcestershire sauce, salt, and pepper. Cook for 2 minutes.
Sprinkle flour over the meat and stir. Add beef broth and bring to a simmer. Cook until slightly thickened, about 5 minutes.
Stir in the frozen peas and carrots. Remove from heat.
For the mashed potatoes: Boil the potatoes until tender, about 15 minutes. Drain and return to the pot.
Add milk, butter, garlic powder, salt, and pepper to the potatoes. Mash until smooth.
Spread the meat mixture in a baking dish. Top with the mashed potatoes, smoothing the surface.
Bake for 30-35 minutes, or until the potatoes are golden and the edges are bubbling.
Let rest for 10 minutes before serving.