Beef Wellington with Béarnaise Sauce

I've always loved the idea of Beef Wellington - lovely, tender beef wrapped in pastry. Seems ideal, doesn't it? One problem: I don't like mushrooms (one of the key ingredients). I've never liked them. They taste like dirt and they're slimy. For the sake of making a proper Beef Wellington, however, I put aside my prejudices and chose to trust my faithful food guru, Michel Roux - mushrooms and all.