1. In a large saute pan, heat the oil until it shimmers. Add the onions and cook slowly, stirring occasionally, until very soft and browned, about 20 minutes. Add the garlic and cook two more minutes. Season well with salt and fresh ground pepper, add Aleppo pepper or paprika, and stir. Remove from the heat.
2. While the onions are cooking, place the spinach in a colander and press the excess water out with your hands or the back of a spoon.
3. In a mixing bowl, beat the eggs with the cream.
4. Roll out the puff pastry to fit rimmed cookie sheet or other ovenproof rimmed baking dish. Place the puff pastry in the dish and press from the center toward the edges to create a rim with the dough.
5. Spread the spinach over the crust. Sprinkle the grated cheddar cheese evenly over the spinach. Pour the egg and cream mixture carefully to just cover. Spread the onions over the entire top, and sprinkle with grated Parmesan and some fresh black pepper.
6. Bake in the lower third of the oven (to ensure the crust cooks well) at 350 degrees until the custard is set and lightly browned, and the crust is well cooked, about ½ hour.
7. Serve warm or at room temperature. If serving the next day, let tart cool before wrapping and refrigerating. Reheat for 10-12 minutes in a 300-degree oven.
Note: This is especially easy if you start with prepared puff pastry - a household essential in the holiday season. Dufour Pastry Shop makes an excellent all-butter dough, available at local Whole Foods stores in the freezer case.
Per serving (based on 8): 527 calories, 13 grams protein, 28 grams carbohydrates, 3 grams sugar, 41 grams fat, 124 milligrams cholesterol, 359 milligrams sodium, 2 grams dietary fiber.
