Mum's Cottage Pie

Place potatoes into a large pot and cover with salted water; bring to a boil. Reduce heat to medium-low and simmer until tender, about 20 minutes. Drain and mash with milk and butter. Season with salt and pepper.

Preheat the oven to 350 degrees F (175 degrees C).

Heat oil in a large skillet over medium heat. Add onion; saute until translucent. Add ground beef; cook, breaking up lumps with a wooden spoon, until browned, 5 to 7 minutes. Add sherry; bring to a boil. Reduce heat to low.

Season the beef mixture with salt and pepper, and stir in Worcestershire sauce. Sprinkle in flour and mix well until mixture is thick and clings together, 1 to 2 minutes more.

Pour beef mixture into the bottom of a casserole dish. Spread mashed potatoes evenly across the top. Season with salt and pepper. Scatter 1/2 cup plus 1/3 cup Parmesan cheese on top. Press a fork into the edges of the casserole to make a pattern.

Bake, uncovered, in the preheated oven until piping hot and cheese is melted down, about 30 minutes.