Preheat Oven: Preheat your oven to 350°F (175°C). Grease a souffle dish or a baking dish and set it aside.

Prepare Ingredients: In a skillet, melt the butter over medium heat. Add the diced onion and fennel, sautéing until they are softened and lightly golden. Add the cooked turkey and cook for a few minutes until heated through. Season with salt and pepper. Set aside to cool slightly.

Prepare Souffle Base: In a mixing bowl, combine the bread cubes, turkey mixture, and grated gouda cheese.

Prepare Egg Mixture: In a separate bowl, whisk together the egg yolks and milk or cream. Season lightly with salt and pepper. Pour this mixture over the turkey and bread, ensuring it’s evenly coated.

Whip Egg Whites: In another clean, dry bowl, beat the egg whites until stiff peaks form. Gently fold the whipped egg whites into the turkey mixture, being careful not to deflate the mixture too much.

Bake: Pour the combined mixture into the prepared baking dish or souffle dish. Place it in the preheated oven and bake for about 30-35 minutes or until the souffle is puffed up and golden brown on top.

Serve: Remove the souffle from the oven and serve immediately while it’s puffed up. It will deflate slightly as it cools.