Boil potatoes and cauliflower until fork-tender, then mash with milk, butter, parmesan, garlic powder, onion powder, and salt.
In a large oven-safe skillet, sauté onions with salt, then add mushrooms and carrots, cooking until tender. Brown ground beef with salt, garlic, and onion powders, then add broth, tomato paste, Worcestershire, and rosemary. Simmer for 8-10 minutes.
Stir in frozen peas, then spread mashed potatoes and cauliflower over the filling. Bake for 25-30 minutes until golden.
Let rest for 5-10 minutes before serving.
Notes
You can broil the top for 2-3 minutes for extra browning.