In a saucepan, add oats, milk, sugar and salt, and bring to a simmer over MED-HIGH heat. Once it’s simmering, reduce heat to MED/MED-LOW and cook, stirring occasionally, until mixture is thickened to an oatmeal or porridge consistency. This should take about 15-20 minutes.
While cooking the oats, preheat oven to 350 degrees F. Butter a 2-quart baking dish or 9-10” cast iron skillet and set aside.
Remove oatmeal mixture from heat and let cool slightly.
Quickly stir the egg yolks and lemon zest into the oatmeal. Stir in the berries.
In a mixing bowl, add the egg whites and beat on MED speed with a hand mixer for about 3-5 minutes, or until semi-stiff peaks form.
Carefully and gently fold the egg whites into the oatmeal until just combined.
Transfer mixture to prepared baking dish/skillet and bake 30 minutes, until golden brown and puffed up. Souffle will sink after pan is removed from the oven, it’s okay, it’s supposed to do that.
If desired, sprinkle with powdered sugar, drizzle with maple syrup and garnish with additional berries.