In a medium saucepan, combine the water, steel-cut oats, barley, and cracked wheat.
Bring the mixture to a boil, then reduce the heat and simmer, uncovered, for 8 minutes. The grains will still be firm.
Pour the grain mixture into a medium bowl and let it cool for about 5 minutes.
Stir in the yogurt, milk, honey, apple pie spice, and salt until well combined.
Cover the bowl and refrigerate for at least 12 hours or up to 3 days to allow the grains to soften and the flavors to meld.
When ready to serve, gently reheat the cereal in a saucepan over low heat until warmed through.
Stir in the chopped apple and dried fruit.
Divide into bowls and sprinkle with toasted nuts.
Add a pinch of cardamom or a dash of vanilla extract along with the apple pie spice for a more complex flavor profile.