public void parallelBranchStart(@Nonnull FlowNode parallelStartNode, @Nonnull FlowNode branchStartNode, @Nonnull ForkScanner scanner) {}

node {
   stage ('test') { 
     echo ('Testing'); 
     parallel nestedBranch: {
       echo 'nested Branch'
       stage('nestedBranchStage') {
         echo 'running nestedBranchStage'
         parallel secondLevelNestedBranch1: {
           echo 'secondLevelNestedBranch1'
         }
       }
     },
    failFast: false
   } 
}