How to get the object from Mono<Object>

Question

I am using WebClient to communicate with other microservices. I want to get user from a remote UserService by id then I want to send this user information to another remote service the problem is I get the response as Mono but I have to get it as User object. i tried block() but it didn't work it always throws this error :java.lang.IllegalStateException: block()/blockFirst()/blockLast() are blocking, which is not supported in thread here is a code

    public User findUserById(String user_id){
        return webClientBuilder.build().get().uri("http://localhost:7854/"+user_id)
                .retrieve().onStatus(httpStatus -> HttpStatus.NOT_FOUND.equals(httpStatus),clientResponse -> Mono.empty())
                .bodyToMono(User.class).block();
    }

I am using Spring Boot 2.5.4 Java 11