Peel and core the apples, cut into 1 cm slices. Melt the butter in a large skillet over medium heat.
Add apples and 1 tablespoon of maple syrup. Fry until apples are tender, about 5 minutes. Add remaining ½ cup maple syrup and cinnamon. Simmer for about 1 minute.
Prepare porridge. Bring water and milk in a large saucepan to a simmer over medium heat. Heat butter in a medium skillet over medium heat until frothy. Add oatmeal and fry, stirring constantly with a wooden spoon, until golden brown and aromatic, 1½ to 2 minutes. While stirring, pour the toasted oatmeal into a saucepan with boiling liquid, reduce heat and cook until the porridge thickens, about 20 minutes. Add salt and stir with a spoon. Continue cooking, stirring occasionally with a wooden spoon, until the oatmeal has absorbed most of the liquid and swelled, 7 to 10 minutes.
Remove from heat and let the porridge stand for 5 minutes. Serve immediately with fried apples.