Peel and grate one medium apple directly into a bowl. The grated apple will cook into the porridge and almost disappear, leaving behind natural sweetness and a beautiful texture. Slice the remaining half apple thinly for the topping and set aside.
Pour the water into a medium saucepan and bring to a gentle simmer over medium heat. Add the rolled oats and grated apple and stir immediately. Reduce the heat to low and cook for 8–10 minutes, stirring regularly, until the porridge is thick and creamy. The grated apple will soften completely into the oats — this is exactly what you want.
Remove the pan from the heat. Stir in the ground cinnamon, ground flaxseeds, and vanilla extract. Mix well — the flaxseeds will thicken the porridge slightly as they absorb moisture, giving it an even creamier consistency. Let it sit for one minute before serving.
Divide the porridge between two bowls. Arrange the thinly sliced apple on top, sprinkle with a pinch of cinnamon and the remaining flaxseeds. If you're using honey, drizzle just a small amount over the top — a little goes a long way here. Serve immediately while warm.