Put the porridge, half of the cinnamon and milk in a saucepan. Bring to a simmer and cook for 10 minutes over low heat. It should be creamy, so add a little more milk, if needed.
Turn off the heat, stir a bit, then put a lid on and rest for 5 minutes.
Meanwhile, peel the grapefruit, and remove the membrane.
Heat the butter in another pan, add the grapefruit flesh, as wedges or torn into pieces, then add the sugar and cinnamon. Caramelize for a few minutes, until sticky.
Spoon the porridge into bowls add the caramelized grapefruit and some nuts, if used. Drizzle with agave/maple syrup and serve.