Toast Steel Cut Oats: Heat up your pressure cooker over medium heat (Instant Pot: press Saute button). Melt 1 tbsp (14g) unsalted butter in the pressure cooker and toast 1 cup (160g) steel cut oats until fragrant (~ 2 minutes).
Pressure Cook Steel Cut Oats: Add 2 roughly sliced gala apples, 1 tsp (5ml) vanilla extract, 1 cinnamon stick, ½ tsp nutmeg, ½ tsp allspice, ½ tsp grated fresh ginger, and a pinch of salt in the pressure cooker. Give it a quick mix and pour in 2 ½ cups (625ml) cold water. Make sure all the steel cut oats are submerged in the mixture and nothing is sticking to the bottom. Close lid and pressure cook at High Pressure for 10 minutes, then Full Natural Release (~12 minutes). Open the lid carefully.
Thicken Oatmeal: The oatmeal will look watery when you first open the lid. Give it a few quick stirs with a silicone spatula to break down the apple slices and thicken the oatmeal.
Serve: Sweeten the apple oatmeal accordingly with your choice of sweetener (honey/maple syrup/brown sugar). Serve with thinly sliced apple. Drizzle more honey or maple syrup on the oatmeal if desired.