Preheat the oven to 375°F (190°C) and thoroughly grease a 9-inch square baking dish.
In a large mixing bowl, combine the rolled oats, 1/2 cup of the sliced almonds, ground cinnamon, baking soda, and kosher salt. Stir until dry ingredients are evenly blended.
In a separate medium bowl, whisk together the milk, sweetening agent (honey or maple syrup), eggs, melted fat (butter or coconut oil), and vanilla extract until completely emulsified.
Pour the wet mixture into the dry oat base, add 2 cups of the frozen mixed berries, and fold until evenly incorporated.
Transfer the combined mixture into the prepared baking dish, leveling the surface with a spatula. Top with the remaining 1/2 cup of frozen berries and sprinkle the remaining 1/4 cup of sliced almonds evenly across the surface.
Bake for 40 to 45 minutes, or until the center is firm to the touch and the edges are lightly golden brown.
Remove from heat and allow to set briefly prior to portioning and serving.