Hi Fishy,
By all means have a go at caramelising your onions but, you would need to do this in very little oil, as using large amounts of oil, makes it very difficult. Also, DO NOT add any salt whilst your doing the onions. Salted onions are impossible to caramelise! And make sure that you don't burn them or it will definitely ruin your base. If it works, you should end up with quite a sweet base!
Good luck,
Ray