Nothing to worry about here, folks: This souffle is the opposite of fussy. Right out of the oven it's gloriously light and airy like a souffle should be, but as it deflates it becomes rich and decadent -- meaning that even if your souffle collapses, it's still going to be amazing.
Place the spinach in a colander to drain, pressing out as much excess liquid as you can.
Make the bechamel sauce: Melt the butter in a medium saucepan over medium heat; add the flour and stir until it becomes a thick paste. Add the milk 1/2 cup at a time, whisking well after each addition, until you have a smooth sauce. Remove from the heat and whisk in the salt, nutmeg and pepper. Taste for seasoning. Set aside to cool slightly.
Beat the egg whites until just frothy with an electric mixer. Add the cream of tartar and beat on high speed until it forms firm peaks. Set aside.
Whisk the egg yolks in a large bowl until they are pale yellow, then add the warm bechamel sauce a bit at a time. Switch to a large rubber spatula; gently mix in the spinach, Parmesan and 1/2 cup Romano. Fold in a third of the egg whites to lighten, then gently fold in the remaining egg whites until thoroughly combined but still light and fluffy.
Lightly grease a 2-quart souffle dish with cooking spray or softened butter, then add the remaining 1/2 cup Romano and rotate the dish to coat the bottom and sides with the cheese. Pour in the souffle batter, then bake until puffy and golden brown, 25 to 30 minutes. Serve immediately.