Prepare the baking dish and heat the oven:
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Mix the ingredients:
Add the rolled oats, milk, eggs, melted butter, honey, yogurt, vanilla extract and salt to a large mixing bowl. Mix the batter until incorporated and cohesive.
Incorporate the berries:
Fold in 3/4 cup blueberries and 3/4 cup raspberries. Evenly distribute the prepared batter into the greased or lined baking dish. Sprinkle the rest of the berries over the top for decoration.
Bake the casserole:
Move the dish to the preheated oven and bake for about 45 minutes. It should be glossy on the surface and firm to the touch when ready. Let it cool a bit before serving.