Instructions
- Caramelize the Onions: Heat Unsalted Butter in a heavy-bottomed skillet over low heat. Add Yellow Onions and cook, stirring occasionally, for 60-75 minutes, or until deeply golden brown and caramelized. Deglaze the pan with Dry Sherry and Balsamic Vinegar, scraping up any browned bits. Cook for another 5 minutes until absorbed. Set aside.
- Prepare the Béchamel: In a saucepan, melt Unsalted Butter over medium heat. Whisk in All-Purpose Flour and cook for 2-3 minutes to create a blond roux. Gradually whisk in Whole Milk, ensuring no lumps form. Bring to a simmer, stirring constantly, and cook for 8-10 minutes, or until thickened and smooth. Stir in Nutmeg, White Truffle Oil, Fleur de Sel, and White Pepper. Remove from heat.
- Combine: Stir the caramelized Onions and 3/4 cup of the Gruyere/Comté cheese blend into the Béchamel sauce. Let cool slightly.
- Prepare the Egg Whites: In a clean, dry bowl, beat Egg Whites with Cream of Tartar until stiff, glossy peaks form.
- Fold and Bake: Gently fold the Egg Yolks into the Béchamel mixture. Then, gently fold in one-third of the Egg Whites to lighten the mixture, followed by the remaining Egg Whites. Pour into a buttered and finely grated Parmesan-coated 6-cup soufflé dish. Sprinkle with the remaining Gruyere/Comté cheese blend.
- Bake in a preheated oven at 375°F (190°C) for 35-40 minutes, or until puffed and golden brown. Serve immediately.
