1. Preheat the oven to 400 degrees F. Butter the inside of a 6 to 8-cup soufflé dish (or six small ramekin dishes) and set aside. If using ramekins, position them on a jelly roll pan or baking sheet.
2. Make a roux: melt the butter in a small saucepan over low heat. With a wooden spoon, stir in the flour and cook, stirring constantly, for 2 minutes. Off the heat, whisk in the hot milk, the nutmeg, 1/2 teaspoon salt and 1/4 teaspoon pepper. Cook over low heat, whisking constantly, for 1 minute, until smooth and thick.
3. Remove from heat and before mixture cools, whisk in the egg yolks, one at a time. Stir in the mozzarella, 1/4 cup of Parmesan, artichokes and spinach and transfer to a large mixing bowl.
4. Put the egg whites, cream of tartar, and a pinch of salt in the bowl of an electric mixer fitted with the whisk attachment. Beat on low speed for 1 minute, on medium speed for 1 minute, then finally on high speed until they form firm, glossy peaks.
5. Whisk one quarter of the egg whites into the cheese sauce. Once blended, fold in the rest. Pour into the soufflé dish, then smooth the top. Draw a large circle on top with the spatula to help the soufflé rise evenly, and place in the middle of the oven. Turn the temperature down to 375 degrees F. Bake for 30 to 35 minutes until puffed and brown.
*NOTE: Recipe created on behalf of Panera Bread by Maris Callahan, InGoodTaste. This is similar to Panera's Spinach & Artichoke Egg Soufflé.