I learned this from a sugar-free website:
In a microwaveable bowl, put 1/3 cup of quick oats and 3/4 cup of milk. (The original recipe suggests adding a spoonful of nut butter for extra protein, but it works fine without it.) Cover loosely with plastic wrap. Cook on High for about 2 1/2 minutes; you're going to have to try this a couple of times with your own microwave to get it right. Watch it during the last minute or so; when you see it poof up as if it's going to boil over, it's done, turn it off. I usually let it sit a few more minutes while I do other things.
UPDATE: I figured out you can use large-flake oats for this too, and it's not as gummy. Just give it slightly longer to cook.