uniqueresult.cs

public virtual T GetByExpression<T>(Expression<Func<T, bool>> expression) where T : class 
{
    var session = _sessionManager.OpenSession();

    var result = session.QueryOver<T>()
        .Where(expression)
        .List()
        .FirstOrDefault();

    return result;
}

This should do what you are after...

public virtual T GetByExpression<T>(Expression<Func<T, bool>> expression) where T : class 
{
    var session = _sessionManager.OpenSession();

    var result = session.QueryOver<T>()
        .Where(expression)
        .Take(1)
        .SingleOrDefault();

    return result;
}