I use an "overnight oats" method that I feel would probably be even more energy-efficient: place the required amount of rolled oats in a plastic container, pour

I use an "overnight oats" method that I feel would probably be even more energy-efficient: place the required amount of rolled oats in a plastic container, pour in the required amount of milk/water, close the lid and store in the fridge overnight. In the morning, heat the mix for a couple of minutes in the microwave (you're just aiming to heat it up at this point). I wouldn't stake my life that this method is more efficient, but it feels like it should be (microwaves are a pretty efficient method of heating a small volume). Takes a bit of forward planning, though.