Moist and light as a feather, this soufflé takes the concept of vegetables to a new level. Even professed spinach-haters like this dish!Many cooks are hesitant to try their hand at making soufflés, but there's nothing mysterious or difficult about the process. If you know how to beat and fold egg whites into a base, your soufflé will be a success. Perhaps the most difficult part of making a soufflé is knowing when it should be taken out of the oven. Use your nose as a guide and remember that a soufflé is meant to be very moist, not dry. Even if your soufflé falls before it gets to the table, it will still taste delicious!
Butter a 1-1/2-quart soufflé dish. Dust the sides and bottom lightly with the 3 tablespoons of parmesan; set aside.
To make the bechamel sauce, melt the 3 tablespoons of butter in a pan over medium heat. Whisk in the flour and cook, stirring constantly, for 2 minutes. Add the milk all at once and whisk to blend. Add the salt, pepper, and nutmeg, and cook until the sauce thickens, 2 to 3 minutes.
Add the spinach to the hot bechamel and cook until it wilts, about 3 minutes. Whisk in the egg yolks and cheese, then remove from the heat and transfer the mixture to a large bowl. Let cool for at least 15 minutes.
Position a rack in the lower middle of the oven and preheat to 375 degrees F.
Place the egg whites in a clean bowl and beat with an electric mixer until foamy. Add the cream of tartar and continue beating until the whites form soft and shiny peaks. Don't overbeat or the whites will be stiff and dry.
Stir one-quarter of the egg whites into the spinach base to lighten the mixture. Quickly fold in the remaining whites. Don't worry if the whites are not evenly distributed: it is better to have some streaks rather than deflate them.
Transfer the mixture to the prepared pan. Place on a baking tray in the oven and cook until the soufflé has puffed and risen 1 to 2 inches above the rim of the dish, 25 to 35 minutes.
When a soufflé is cooked, you can generally tell because the aroma of it permeates the room. The soufflé should still be moist in the center but firm around the edges. Avoid the temptation to open the oven and check on it, as you will lose the heat and the soufflée may fall. Serve immediately.