How to use custom Spring EL function in Thymeleaf?

Not in front of a way to test, but offhand try using the static call:

th:text="${T(com.package.SUtils).reverseString('hello')}"

What I usually do, is to define Thymeleaf utility classes like this as a Bean using @Component. In Spring EL you can then simply refer to them using @ with auto-detection. So there is no need to register them.

@Component
public interface SUtils {

  static String reverseString(String input) {
    // ...
  }
}

---

<span th:text="${@sUtils.reverseString('hello')}"></span>