Place the ricotta in a strainer lined with cheesecloth and allow to drain over a bowl, covered and refrigerated, for 2 hours. To prepare the tart dough, combine the flour and 1/2 teaspoon of the salt in a bowl. Add the chilled butter and, using your fingertips or two knives, blend until the mixture resembles coarse meal. Work in the ice water until the dough holds together. Form the dough into a smooth, flat disk, wrap in plastic, and refrigerate for at least 1 hour. Lightly flour a clean work surface and roll the dough into a 16inch disk. Place the rolled out dough into a 10- by 2 1/2-inch deep cake pan or springform mold. Pressing with your fingertips, flute the top edges of the dough. Place in the freezer and chill thoroughly, about 30 minutes. In a medium skillet, heat 1 tablespoon of the olive oil over high heat and saute the spinach, stirring constantly until wilted, about 2 minutes. Season with 1/4 teaspoon salt and half the pepper. Place the cooked spinach in a colander and drain. Chop fine and reserve. In the same skillet, add the remaining tablespoon olive oil and cook the onion 3 to 5 minutes over medium heat until translucent. Add the marjoram and prosciutto. Stir and cook an additional 2 minutes. Set aside. In a large bowl, beat the eggs and add the drained ricotta, spinach, onion, and prosciutto. Season with the nutmeg and remaining salt and pepper. Set aside. Line the tart shell with aluminum foil and fill with dried beans or pastry weights. Bake for 12 to 15 minutes, until the dough is set. Remove the foil and beans and cook an additional 10 minutes, or until the dough is light brown. Lower the oven temperature to 375 degrees. Spread the ricotta and spinach mixture evenly in the tart, place on the middle rack of' the oven, and bake for 1 hour and 10 minutes, until the filling is set and the top golden brown. Let rest for 15 to 20 minutes, slice into wedges, and serve.
Recipe Courtesy of Chef Michael Romano, author of The Union Square Cookbook