I have a crockpot that I use for my cholent. Is my wife allowed to put soup that consists of mostly solid into a piece of foil, and then put the foil on top of the glass cover of the crockpot.
Heating the soup up on Shabbos is problematic because the liquid soup may not get reheated, since the liquid soup will get reheated, even tough most of the soup is solid pieces. This would apply even your intention is to remove it from the heat source before it actual gets hot. If however the soup’s being on the top of the crock pot cannot get hot (yad soledes bo) over there, then it would be permitted.
M:B 318- 32,39, Shar Hatzion 60, Shevet Halevi 11-67, Orchos Shabbos chap.1- 22.