Make sure to get a nice sear on your beef but leave it pretty rare. Let it rest and cool down a bit before you try to work with it. Your mushroom mixture needn't be "busy" or complicated; basically just chop 'em finely (or use a food processor). Don't be afraid to chop them almost to a paste. Sautee them with a tad of butter and S&P, then add a touch of white wine. Then cook them til nearly all the moisture is gone. Let it cool off before you use it.
Beyond that, you can make a large Wellington or several individual ones. I think the latter is a bit simpler from a service standpoint. Wrap them tightly, score each one a bit to let the steam out, and be sure to seal them with a beaten egg.
Hope they work out well for you.
If we're not supposed to eat animals, then how come they're made out of meat?