Preheat the oven to 450 degrees F.
Pour the olive oil into a preheated 8-inch cast-iron pan over medium-high heat. Allow the oil to heat up, and then add the ground beef
and some salt. Press the meat into the pan and cook until caramelized, about 5 minutes, and then flip and caramelize
the other side.
While the beef cooks, begin the mashed potato topping. Fill a large pot with cold water and add the potatoes, cauliflower
and a generous pinch of salt. Bring to a boil, then reduce to a simmer
and cook over moderate heat until tender, about 20 minutes.
When the beef is dark brown and caramelized, remove it from the pan and set aside. Add the celery, celery leaves and onions
and let sweat for 1 minute. Add the pine nuts and tomato paste
and cook for another minute or two. Stir in the cinnamon, garlic and orange zest
to combine. Return the caramelized beef to the pan and stir in the parsley
. Add a splash of water, if needed. Remove from the heat and set aside.
When the potatoes and cauliflower are done cooking, drain
and pass through a food mill
or potato ricer
. Stir in the Parmesan
and butter until smooth and uniform.
Spread the potato mixture over the beef mixture in the pan, and smooth the top to seal in the beef.
Bake until golden brown, 10 to 15 minutes.