Instructions: In a medium skillet, fry the chopped bacon until crisp and remove with a slotted spoon to a paper towel-lined saucer. Set aside. Have oven heating to 450 F (this is extraordinarily high for a delicate egg dish, but the author/chef said the recipe was tested and re-tested at this very hot temperature). Butter a 2-quart souffle dish and dust with half the Parmesan cheese. Add a foil collar to contain the volume from an extra egg white: Measure out a sheet of foil long enough to encircle the souffle dish with an extra 2 inches left over and fold it in half lengthwise. Butter one side of the foil. Wrap it, butter side in, around the souffle dish, positioning it so that it stands 2 inches above the rim of the dish. Secure in place with kitchen string. In a medium saucepan over medium heat, melt the 2 tablespoons of butter. Saute the onion, stirring often, about 5 minutes.

Stir in the flour and cook, stirring constantly, about 2 minutes, allowing mixture to bubble. Meanwhile, gradually whisk in the milk over medium heat and cook until the sauce is thickened and smooth except for the onion.

Transfer sauce to a large bowl and stir in the bacon. Then add the egg yolks, one at a time, stirring well after each addition . Season with salt and pepper and set aside. In a large mixer bowl, metal or glass, beat the egg whites until stiff peaks form. Using a rubber spatula, fold half the whites into the yolk mixture to lighten it. Top with Gruyere or Swiss, the remaining whites and the spinach. Fold in just until no white drifts remain. Do not overmix and beat out the air. Pour into the prepared souffle dish and sprinkle the remaining Parmesan on top. Bake until the top is golden and the center no longer quivers when the dish is shaken, 45 minutes. Remove foil and serve immediately, dividing portions with two forks.

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