In a large bowl, combine 500g natural yogurt, 60g sugar, and 1 tablespoon lemon juice. Stir well and season to taste.
Wash the 3 apples, remove the cores, grate them, and add to the yogurt mixture. Stir well.
In a separate bowl, whip 100g cream until stiff peaks form, about 2-3 minutes.
Fold the whipped cream into the yogurt-apple mixture. Refrigerate for at least 10 minutes.
Melt 20g butter in a pan over medium heat, about 1 minute.
Add 20g sugar, 5 tablespoons chopped almonds, and 5 tablespoons crunchy oat flakes. Stir constantly over medium heat until crispy and golden, about 3-4 minutes.
Pour the roasted mixture into a bowl and let cool for 5 minutes.
Divide the chilled yogurt mixture among small bowls and top with the roasted almonds and oat flakes.