Cooking Buckwheat without the smell
The very best option would be to cook sous vide; that way all of the aroma will stay in the bag. You don't need any special equipment. We don't have a recipe for grains specifically, but there is a good, free intro to cooking sous vide with just ZipLoc bags and a pot on the stove here: https://www.chefsteps.com/classes/sous-vide-cooking/landing#/

If that doesn't work for you, a non-venting style pressure cooker should help some, as once it reaches pressure it won't release any aroma during cooking. You could even let it cool naturally and then open it outside and transfer to a sealed container.