Can I make instant oatmeal on Shabbos?
There are two Halachic issues with making instant oatmeal on Shabbos:
1. Bishul: Since the oatmeal was never cooked before, even if it was already baked, one is still not allowed to cook it. Many maintain that this applies even if using a Kli Shlishi.
2. Lash: When pouring the water and mixing it, it is considered making a dough.
If you pour a lot of water at once, so that it is not a thick mixture. and you make Shinui both in the order of placing the ingredients and in the method of mixing it, it would be permitted. However, in our case it is still not permitted because of the issue of “Bishul” as mentioned in number 1.
שו״ע אדה״ז או״ח סי׳ שיח יא. ובמ״ב ס״ק לט׳ מדין קולית האספנין. שבת כהלכה א ע׳ פג.
שו״ע או״ח סי׳ שכא טז. שו״ע אדה״ז שמ, טז. שבת כהלכה כב, כז. שם נח.