In a small saucepan pan over medium heat, add in butter and steel cut oats. Toast the oats, stirring frequently with a spatula for three minutes.
Turn the heat to low and add in the water. Cover the pot and let simmer, stirring occasionally, until the steel cut oats have soaked in the water, about 20-25 minutes. If you prefer a thinner consistency, add in more water as needed.
Stir in the milk and cook for an additional five minutes.
Cut the plums into slices about 1 centimeter thick. Leave on the skin but discard the pit.
Toss the plums in the demerara sugar until well coated.
In a small pan over medium-high heat, melt the butter.
Sear plum slices for about five minutes, turning once gently. It's beautiful if you place some plums skin-side down on the pan so the skin gets browned.
Serve the seared plums on top of the porridge. Top with additional milk and brown sugar as desired. Enjoy immediately.