I have the following function

public ReturnVal doSOmething(String value){
     List<String> ids = Arrays.asList("val1", "val2", "val3");

     List<Root> roots = new ArrayList();
     List<Chain> chains= new ArrayList();

     Root rootValue = new Root(value);
     for(String id : ids){
        RootBuilder builder = new RootBuilder();
        builder.add(value, id);
        String newRoot = builder.build();
        roots.add(newRoot);

        ChainBuilder chainBUilder = new ChainBuilder();
        chains.add(builder.build(newRoot));
        rootValue = new Root(newRoot.value(), newRoot.level());
     } 

     Signature signature = signingService.sign(rootValue.value(), rootValue.level()); // Blocking, needs to connect to a server through TCP. Server needs a fix before webClient can be used.
     SigntureFactory factory = new SignatureFactory();

     for(int i = chains.size()-1; i >= 0; i--){
       Root root = roots.get(i);
       signature = factory.createSignature(signature, chains.get(i), root.value())

    }
    return signature;
}

What it does is that it signs some data (its a custom signing system) and then afterwards adds a chain of custom ID-s to the signature.

What i cant figure out is can this be somehow made into a java reactive setup somehow? Builders and factory are a API classes, i cant modify them.
This method is called when a REST endpoint is invoked.

I could proably wrap signingService.sign into a Mono.just or Mono.fromSupplier But the loops seem like Flux but i need to return Mono in the end.