Instructions
- Caramelize the Onions: Heat Olive Oil in a large skillet over medium-low heat. Add Yellow Onions and cook, stirring occasionally, for 45-60 minutes, or until deeply golden brown and caramelized. Set aside.
- Prepare the Béchamel: In a saucepan, melt Butter over medium heat. Whisk in All-Purpose Flour and cook for 1-2 minutes to create a roux. Gradually whisk in Milk, ensuring no lumps form. Bring to a simmer, stirring constantly, and cook for 5-7 minutes, or until thickened. Stir in Nutmeg, Salt, and Pepper. Remove from heat.
- Combine: Stir the caramelized Onions and 1 cup of Gruyere Cheese 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 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 6-cup soufflé dish. Sprinkle with the remaining Gruyere Cheese.
- Bake in a preheated oven at 375°F (190°C) for 30-35 minutes, or until puffed and golden brown. Serve immediately.
