Controller problem with header's data with Spring Boot and Thymeleaf

so here's my problem. I'm trying to have dynamic data in my header. It's supposed to represent the number of items a user has in his cart. So here's my header (the header is a fragment that I included in the layout.html) and the data is in the last li

<nav >
            <ul>
                <li><a th:href="@{/index}" class="">Home</a></li>
                <li><a th:href="@{/products/list}" class="">Products</a></li>
                <li><a th:href="@{/admin/index}" class="">Admin</a></li>
                <li><a href="" class="">Account</a></li>
                <li><a href="" class="">Contacts</a></li>
                <li><a th:href="@{/login}" class="">Login</a></li>
                <li><a th:href="@{/user-page}" class="">User</a></li>
                <li>
                    <p th:text="${cart.id}"></p>
                </li>
            </ul>
</nav>

But since the header is available on every page of the web app, what should I do (in the controllers) to get this data? I tried to put the cart data in the IndexController but it doesn't work. I'm not comfortable with Spring and Thymeleaf so I might be doing something wrong or maybe I missed something on data binding. I only have once constraint, I don't want to use Ajax or Jquery. Unless I have no choice but I doubt that. Here's what I tried in the IndexController

@GetMapping({"/", "/index", "/home"})
    public String getIndex(Model model) {
        this.userCart.setId(5);
        this.userCart.setNumberItems(11);
        model.addAttribute("cart", userCart);
        model.addAttribute("phrase", "Oui oui si si 92izi");
        model.addAttribute("nbItems", "10");
        return "index";
    }

I also tried to make another method mapped to "/" but it causes an error since I can't have to methods mapped to the same url. So what am I supposed to do ?