Place the milk, maple syrup, vanilla and cardamom into a saucepan and bring to a boil. Turn off the heat and leave the pot on the hot plate.
Add the instant oats to the hot milk and give it a quick stir. Let it sit for about 3 minutes.
Just before serving add the 3 tbsp rolled oats and frozen berries and stir just until combined before pouring into a bowl. I like to add rolled oats just in the end so they absorb the remaining liquid and give the porridge a more pleasant texture.
Scatter the pistachios on top and drizzle with a bit of maple syrup if you want.