Who is the Italian soprano praised by Maria Callas and if possible, on
what occasion?

Thanks

munquai --

Could you be thinking of Renata Tebaldi?  See this excerpt from Wikipedia:

"There is said to have been a rivaly between Tebaldi and Maria Callas,
but it appears to have been more a rivaly between the highly partisan
fans of the two famous divas. While both where active in Italy in the
1950s, they had very different voices, different reperoires, and
different tempraments. In any case, on September 16, 1968, Callas went
backstage after a performance of Adriana Lecouvreur to warmly
congratulate Tebaldi, thereby dispelling any bad blood once and for
all."

Wikipedia: Renata Tebaldi
http://en.wikipedia.org/wiki/Renata_Tebaldi

markj-ga

Thanks for your comment but it is definitely not Tebaldi.
munquai