Wash the half apple and remove the core. Cut the apple into small, uniform cubes with a knife. Scoop 500g of full-fat Greek yogurt into a large bowl and gather the remaining ingredients within reach.
Add the 2 tablespoons of broken flaxseeds, 1 teaspoon of cinnamon, and the fresh apple cubes to the bowl with yogurt. Stir everything together firmly with a spoon until the additions are evenly distributed throughout the thick mixture.
Finish the preparation by sprinkling a pinch of Celtic sea salt over the yogurt mixture. Fold everything together one last time to ensure the salt is well incorporated into the yogurt.
Divide the finished yogurt among serving bowls or breakfast cups. Serve immediately so that the apple cubes retain their firm structure.