How to thinly slice chicken thighs?
You are certainly correct to cut against the grain.
Certainly slicing meat would be difficult with a dull knife, but I have no way of knowing how sharp your knife is from reading your question. While &quot;sharpening&quot; a knife with a ceramic mug may be a neat trick in a pinch, you should look into a better solution. Using a whetstone is often recommended, or you could pay someone else to sharpen your knife occasionally. Having a reliably sharp knife is essential for precision and safety in the kitchen, and will make a lot of tasks easier.
Once you get your knife sorted, the usual trick to slicing meat thinly is to partially freeze it before cutting. These instructions from Japanese cooking website Just One Cookbook go into more detail about slicing meat thinly, but are primary aimed at large pieces of beef or pork. But the basic idea still works. Chicken thighs are on the thin side, so about 15-20 minutes before slicing, place them, flattened and wrapped, in the freezer. Once removed, they will be just firm enough to cut precisely. If they are too firm, let thaw briefly; too soft, freeze further.
Now, this is a matter of person preference, but for oyakodon specifically, I do not like the chicken cut too thinly. Paper-thin chicken overcooks easily, turning mealy and sometimes even breaking up in the broth. I cut my chicken to between 1/4 and 1/3 of an inch (0.6 to 0.8 cm) thick, cutting at a bias for more surface area. At this thickness, I can often get good slices without freezing.