C# - How do I define an inline method Func<T> as a parameter?

public static T GetItem<T>(string key, Func<T> defaultValue)
{
    if (HttpContext.Current.Session[key] == null)
    {
        HttpContext.Current.Session[key] = defaultValue.Invoke();
    }
    return (T)HttpContext.Current.Session[key];
}

Foo foo = GetItem<Foo>("abc", () => new Foo("blah"));

return ((T)HttpContext.Current.Session[key]) ?? defaultValue();

public static T GetItem<T>(string key)
    where T : new()
{
    return ((T)HttpContext.Current.Session[key]) ?? new T();
}

var log = SessionItem.GetItem("logger", () => NullLog.Instance)