@RestController
public class Controller
{
    @PostMapping("/doStuff")
    public Mono<Response> doStuff(@RequestBody Mono<Request> request)
    {
        ...
    }
}

@PostMapping("/doStuff")
public Mono<Response> doStuff(@RequestBody Mono<Request> request)
{
    return request.flux()
                  .groupBy(r -> r.someProperty())
                  .flatMap(gf -> gf.map(r -> doStuff(r)));
}

private AtomicReference<FluxSink<Request>> sink = new AtomicReference<>();
private Flux<Response> serializingStream;


public Controller()
{
    this.serializingStream =
        Flux.<Request>create(fluxSink -> sink.set(fluxSink), ERROR)
            .groupBy(r -> r.someProperty())
            .flatMap(gf -> gf.map(r -> doStuff(r)));
            .publish()
            .autoConnect();

    this.serializingStream.subscribe().dispose(); //dummy subscription to set the sink;
}

@PostMapping("/doStuff")
public Mono<Response> doStuff(@RequestBody Request request)
{
    req.setReqId(UUID.randomUUID().toString());
    return
        serializingStream
            .doOnSubscribe(__ -> sink.get().next(req))
            .filter(resp -> resp.getReqId().equals(req.getReqId()))
            .take(1)
            .single();
}