Cut the grapefruits in half horizontally, and cut the flesh from the skin with a grapefruit knife.
Put the Florida grapefruit parts in the blender.
Now, peel the skin from the frozen banana with a peeler, cut the banana into pieces and, together with the yoghurt, add it to the grapefruit in the blender.
Start the blender and let everything purée, add 4 ice cubes and the oatmeal and blend again until the ice has been crushed.
Serve straight away in tall glasses.
For a dairy-free or vegan version, just use a plant based yoghurt instead of the non-fat yoghurt or kefir.Please note: calories and other nutritional information are per serving. They're approximate and will depend on exact ingredients used.