Combine 2 cups 200 g of rolled oats with 1⅔ cups 400 ml of milk in a bowl, stir and leave to soak for 10 minutes so the oats soften.
Add a pinch of salt and an additional 400 ml of milk to the soaked oats, mix well and leave for a further 5–10 minutes to continue softening.
In a separate bowl, whisk 2 eggs until smooth and slightly frothy, then add 40 g of sugar, an optional pinch of salt and 1 teaspoon of baking powder, mixing thoroughly. Pour the egg mixture into the soaked oats and stir carefully until you have a uniform batter with no dry patches of oats.
Grease a baking dish generously with butter, covering the base and sides to prevent sticking.
Add 50 g of blueberries and the small cubes of 1 apple to the oat batter and fold them in gently so the fruit is evenly distributed.
Pour the batter into the greased baking dish and spread it evenly, levelling the surface with a spatula.
Crumble small pieces of butter over the top of the batter, placing them evenly across the surface.
Bake in a preheated oven at 360∘F 180∘C for about 40 minutes until the pie is golden brown and set in the centre.
Allow the pie to cool slightly in the dish, then slice into portions and serve warm or at room temperature as a tender oatmeal blueberry pie.