2 to 3 tablespoons olive oil
1 1/4 pounds ground beef
2 large russet potatoes, peeled and cut into large chunks
1 head cauliflower, quartered and core removed
1 cup chopped celery plus 1/2 cup chopped celery leaves
2 large onions, chopped
3 tablespoons pine nuts
1 generous tablespoon tomato paste
Pinch ground cinnamon
1 clove garlic
Zest of 1/2 orange (about 1 teaspoon)
1/2 cup chopped fresh parsley
1 cup freshly grated Parmesan
3 tablespoons unsalted butter, room temperature
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.