Preheat oven to 190 C (375 F) and line a loaf tin with baking paper. Leave the edges of the baking paper hanging over the top of the tin, so you can use the baking paper to easily lift out the slice.
Combine oats, berries and dates in a mixing bowl, and stir to combine.
In a separate bowl, whisk egg and milk together.
Add egg and milk to the oat mixture and stir until all the oats are coated with liquid.
Pour into prepared loaf tin and use a spoon to spread the oat mixture out evenly.
There will be a lot of liquid in there - don't worry, this will all absorb into the oats as it bakes.
Bake for approximately 60 - 70 minutes, or until all liquid is absorbed and the slice feels firm.
Remove from oven and let cool for 10 - 15 minutes.
Lift slice out of the tin by holding on to the baking paper, and leave on the bench to cool completely.
Once cool, cut slice into six square pieces.
Wrap each one individually in plastic wrap and store in an airtight container in the fridge or freezer.