In a large bowl, combine the rolled oats with the Greek yogurt and let it sit for 15 minutes to allow the oats to hydrate.

In a blender, combine the diced carrots and apples and blend them. Add the hydrated oats in yogurt and the eggs.

Incorporate the honey or agave syrup, the baking powder, and blend until you get a smooth mixture.

Pour the mixture into a 20 cm (8-inch) square pan lined with parchment paper, level the surface, sprinkle the chopped walnuts on top, and bake in a preheated oven at 350°F (180°C) for 40-45 minutes.

Remove the cake from the oven and let it cool for 10 minutes before transferring it to a wire rack and brushing it with orange marmalade.