Preheat the grill to medium heat. Combine apple, apple juice, nutmeg, and half of the sweetener in a saucepan over high heat. Bring to a boil, then lower heat and simmer uncovered for 15 minutes until the apple softens. Transfer the mixture to a 20cm round ovenproof dish.
In a bowl, mix hazelnut meal, flour, and the rest of the sweetener. Pour in the water and mix until it looks like coarse breadcrumbs. Spread evenly over the apple mixture.
Grill until golden brown under preheated grill, approximately 7cm from the heat source, for about 2 minutes. Serve with a dollop of yogurt.