If you can't find escarole, other sturdy chicories like endive or radicchio can be substituted for a similar flavor. Or, for a milder flavor, try subbing kale. This soup freezes very well; if you choose to freeze, don't add the escarole, and instead add that when you defrost and reheat. If you don't have any parmesan rinds stashed in your freezer, or a piece of cheese it can be trimmed away from, some cheese counters sell rinds for a reasonable price.