Answer for https://stackoverflow.com/q/74346049/1930099

MyController.java

import java.util.List;
import lombok.Data;
import org.springframework.http.MediaType;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
import reactor.core.publisher.Flux;

@RestController
@RequestMapping(produces = MediaType.APPLICATION_JSON_VALUE)
public class MyController {

  @GetMapping("/test")
  Flux<UserDto> dtoFlux() {
    return Flux.just(new UserDto(), new UserDto(), new UserDto())
        .flatMap(userDto -> MyService.findAll()
            .collectList()
            .map(relatedItemsAsField -> {
              userDto.setRelatedItemsAsField(relatedItemsAsField);
              return userDto;
            }));
  }
}

class MyService {

  static Flux<Integer> findAll() {
    // simulates Flux of User related data (e.g. Orders or Articles)
    return Flux.just(1, 2, 3);
  }

}

@Data
class UserDto {

  private List<Integer> relatedItemsAsField;

}