Making savory, light-as-air soufflés is way easier than you might think—seriously—with a few key ingredients and technique secrets.
Position rack in bottom third of oven; preheat to 400°F. Coat six 8-ounce ramekins with 1/2 tablespoon softened butter. Sprinkle with breadcrumbs, tilting and rotating to distribute evenly. Shake out excess. Place the ramekins on a baking sheet and refrigerate.
Heat milk in a microwave-safe measuring cup in the microwave until hot, about 1 1/2 minutes. Melt the remaining 2 tablespoons butter in a small saucepan over medium heat. Add flour and cook, whisking constantly, until a nutty aroma develops, about 2 minutes. Slowly whisk in the hot milk. Bring to a simmer and cook, stirring, until bubbling and thickened, about 2 minutes. Remove from heat. Whisk in nutmeg, salt and pepper.
Place egg yolks in a medium bowl. Gradually whisk in 2 tablespoons of the warm sauce. Slowly whisk in another 2 tablespoons sauce, then whisk in the remaining sauce. Whisk in spinach, Gruyère and Parmesan.
Combine egg whites and cream of tartar in the bowl of a stand mixer fitted with a whisk attachment. Beat on high speed until stiff peaks form. With a flexible spatula, fold one-third of the whites into the soufflé base to lighten it. Carefully fold in the remaining whites in 3 batches until mostly combined (it's OK if a few white streaks remain). Divide the mixture among the chilled ramekins.
Transfer the ramekins, on the baking sheet, to the oven. Reduce the temperature to 375°. Bake until the soufflés are light golden and puffed, about 25 minutes (resist the temptation to peek until the last 5 minutes). Serve immediately.
Six straight-sided 8-oz. ramekins (You can also use twelve 4-oz. ramekins; bake for 15 to 17 minutes.)