We know that the leniency of אין בישול אחר בישול applies to dry, solid food. Therefore, fully cooked food that is dry and solid can be heated on Shabbat in accord with the rules we have learned.
We also know that this leniency of reheating does not apply to דבר לח (a liquid).
But all solid foods have some moisture in them and on them. Exactly how much moisture categorizes a food as liquid?
Sources are available through the links as usual.