The Brockhaus from 1894 supplies this information:
Grütze, grobgemahlenes von den Hülsen befreites Getreide […] Russische
oder braune Grütze, die zuerst in einer eisernen Pfanne trocken
geröstet und dann im Wasser ausgequollen ist, gibt man in Russland zu der
So, as tofro points out, Grütze also means coarsely ground grain, as can be seen by the word Hafergrütze. The second part describing a recipe from Russia, where Grütze is roasted in an iron pan to be added to cabbage soup, would suggest, that your word is composite of Grütze and Pfanne (engl. pan, also pronounced Pann in some German regions). The geographical context seems appropriate, since Keyserling came from Latvia bordering to Russia.
Update: Unfortunately the larger context, which I added to the question, is inappropriate for mentioning kitchen equipment.