Begin by warming your medium saucepan over medium heat. Add the steel cut oats and toast them lightly for 2–3 minutes, stirring frequently to deepen the nutty flavor.
Carefully pour in 4 cups of water along with a pinch of salt. Stir to combine and bring the mixture to a gentle boil over medium-high heat.
Once boiling, reduce the heat to low and simmer uncovered. Stir occasionally to prevent sticking. Cook for about 20 minutes until the oats are tender but still chewy.
Add the diced apple and ground cinnamon to the saucepan. Continue cooking for another 5–10 minutes, stirring regularly, until the apples soften and infuse the oats with flavor.
Remove the pot from heat. Stir in the maple syrup or honey and the milk or milk alternative to enrich the oats and balance sweetness.
Ladle the warm Apple Cinnamon Steel Cut Oats into bowls. Sprinkle with chopped walnuts or pecans for a satisfying crunch. Serve immediately.