I created a method as shown online:
@NonCPS
def parseJsonString(String jsonString) {
    def lazyMap = new JsonSlurper().parseText(jsonString)
    // JsonSlurper returns a non-serializable LazyMap, so copy it into a regular map before returning
    def m = [:]
    m.putAll(lazyMap)
    return m
}
But I get the following error:
ERROR: java.io.NotSerializableException: groovy.json.internal.LazyMap
To work around this, I have to create an entire method to perform an entire step. For instance, in a method, I would do the same as above, parse the information I want, and finally return it as a string.
This, however, presents another issue, especially if you wrap this method inside a withCredentials, which would then require another withCredentials.
 
    