How do you do yours?

We are having an early Christmas celebration this weekend and I am making Beef Wellington. I intend to wrap the fillet in prosciutto before covering with the pastry, but I don't want to use a mushroom duxelles, and I think pate is out too.

So, what can I put on my beef before wrapping it in the ham and pastry, to keep it moist?

has anybody tried caramelised onions/ shallots/ garlic? any other ideas? thanks. (I've been meaning to ask this for ages, but Jacques's appearance has for seem reason been triggering as to this forgotten question.)