To prepare the béchamel sauce, melt the butter in a medium sauté pan over medium-low heat. Once melted, add the Onion and cook, stirring occasionally, for 2 minutes until the Onion is translucent. Add the water and simmer until the water has cooked away and the Onion caramelizes to golden-brown, stirring regularly for about 30 minutes. Add the flour and stir until dissolved. Cook over low heat for 2 minutes, stirring often. Slowly pour the milk into the roux, stirring constantly, until it is fully blended. Simmer while stirring for 10 minutes. Remove from the heat and stir in the nutmeg and salt. Set aside or refrigerate for later use.
Bring 2 quarts of water and 1 tablespoon of kosher salt to a boil in a saucepan. Fill a medium bowl with ice water. Submerge the Spinach in the boiling water for just an instant. Immediately drain and bathe in ice water for a couple of minutes to cool the Spinach. Transfer to a colander and squeeze out all excess water. Chop the Spinach roughly and set aside. To temper the béchamel into the egg yolks, place the lightly beaten egg yolks in a large, stainless steel bowl. Whisk a few drops of the hot béchamel sauce into the egg yolks. Continue whisking as you gradually add half of the hot sauce into the egg yolks. Then, add the last half of the hot sauce, whisking to combine thoroughly. Stir the chopped Spinach into the béchamel-egg mixture; cool down to room temperature and fold in the shredded Gruyère cheese.
Preheat the oven to 425ºF. For appetizer portions, use eight, 3-ounce ceramic ramekins. For main course servings, use four, 6-ounce ramekins. Coat the ramekins with olive oil, dust with a good coating of Parmesan cheese and set aside. Prepare the meringue by whipping the egg whites and cream of tartar together until the meringue reaches a medium-high peak. Fold a third of the meringue into the béchamel-Spinach mixture to loosen it up. Continue to add the remaining meringue by gently blending.
Portion the soufflé mixture into the prepared ramekins. Carefully wipe clean each soufflé dish rim. Sprinkle each soufflé top with Parmesan cheese. Bake 3-ounce ramekins for 20 minutes or 6-ounce ramekins for 25 minutes until golden-brown. Serve immediately. Serves 6 to 8.