In a large mixing bowl, combine the rolled oats, grated apple, milk, yogurt, honey (or maple syrup), and vanilla extract (if using). Mix well until everything is evenly combined.
Stir in the chopped nuts and seeds, reserving a handful for topping.
Cover the bowl with a lid or transfer the mixture to an airtight container. Refrigerate overnight (or for at least 4 hours) to allow the oats to soften and absorb the liquid.
In the morning, give the muesli a good stir. If it’s too thick, add a splash of milk to reach your desired consistency.
Divide the muesli into serving bowls and top with fresh fruit and the reserved nuts and seeds. Drizzle with extra honey or maple syrup if desired.
