There is a concept of ein bishul achar bishul (you can't cook a food more once its cooked already) . This concept applies to dry foods according to all opinions and one would be able to warm such food up without worrying about cooking it. However, with regard to liquids ashkanazim are machmir and hold that there is more cooking that could be done to a liquid.
Many people are aware of this and will not warm up a liquidy food such as a saucy chicken dish on shabbas day. My question is why can't one just pour hot water from an urn (kli rishon) into a kli sheni (empty aluminium pan) and then take the saucy chicken (which is in a smaller aluminum pan) and place it in the kli sheni to warm it up. People do this every shabbas when they pour cold milk into their coffee. Why do people refrain from warming up cold (already cooked) sauce in a kli sheni?
Any time I see this halacha mentioned in a sefer or on a halacha sheet for the rules of warming up, they unequivocally say that warming liquidy foods is assur, but why wouldnt they give such advice?