Mushroom replacement in Beef Wellington
I don't think you can make a Beef Wellington without the obligatory mushrooms, it essentially becomes something else.

The whole concept behind beef wellington is to keep the meat juices in by being surrounded by pastry but there's no reason why you shouldn't omit the mushrooms for something else. I really can't think of any other food substance that would replicate the flavour of mushrooms without being mushrooms other than truffle.

I would try something like a pork/onion sausage meat stuffing and being the time of year it is, perhaps with chestnuts as a possible alternative, either surrounding the beef or inside it.