Prepare the mashed potatoes: Peel the potatoes and cut them into medium-sized cubes. Place them in a large pot and cover with fresh water. Season with sea salt and bring to a boil. Reduce the heat to low, cover, and simmer for about 20 minutes, or until the potatoes are fork-tender.
Preheat the oven: Preheat the oven to 200°C (400°F), with a rack positioned in the center.
Prepare the meat and spinach filling: While the potatoes are cooking, finely chop the onion and roughly chop the spinach. Heat the oil in a large, wide skillet, swirling the pan to coat the bottom evenly. Over high heat, add the minced meat and cook until browned and no pink remains. Stir in the onion and cook for 2 minutes. Add the chopped spinach and continue sautéing until the leaves wilt. Season with sea salt, a pinch of ground cinnamon, and freshly ground black pepper. Set aside.
Mash the potatoes: Drain the cooked potatoes well and mash them using a potato masher or food mill. Add the butter, grated Parmesan cheese, grated nutmeg, freshly ground black pepper, and sea salt. Give everything a quick stir, then add the heavy cream and mix until the mashed potatoes are smooth and creamy. Set aside.
Assemble the Lebanese Potato Soufflé: Grease a large baking dish and spread half of the mashed potatoes evenly over the bottom. Spoon the meat and spinach filling over the potatoes, spreading it into an even layer. Gently spread the remaining mashed potatoes over the filling, making sure to cover it completely.
Adding the top layer can be trickier than it might seem, but here's a little trick worth sharing! Dip a spoon in water and use the back of the spoon to gently spread the mashed potatoes. This helps prevent the potatoes from mixing with the meat filling and keeps the layers neat and distinct.
Add the topping: Beat the egg with a drop of vinegar and generously brush it over the top layer of mashed potatoes. Sprinkle the breadcrumbs evenly over the surface.
Bake: Bake for 20–25 minutes, or until the top is golden brown and crispy. Remove from the oven and let the Lebanese Potato Soufflé rest for 10 minutes. This allows it to cool slightly and the layers to set, making it easier to serve.
Serve: Use a metal spatula to cut and lift out neat portions onto individual plates. Serve warm and enjoy every bite of this comforting Lebanese potato casserole!
Happy eating!