Planning on making for NY Eve and would like to prep as much in advance as possible.
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Lots of results on Google but Martha can (usually) be counted on to have a recipe that makes sense.
I prepped everything the night before I wanted to serve it -- the searing, mustard, duxelles, proscuitto, all of it -- and then left it well-wrapped in cling film in the fridge until the next day. I just had to wrap it in the puff pastry and bake it the day of, and it tasted great to me! Maybe letting it sit at room temperature a bit before wrapping in the puff pastry would have been beneficial, but I don't know about that for sure.
There was just a Food Network segment on that - and in fact, the restaurant made the Wellington 24 hours in advance and kept it in refrigeration until baking. It was an Alton Brown segment.
Fine Cooking has an excellent recipe with make ahead tips for Wellington. And there certainly is no Proscuitto in it. What the heck, whoever thought that up?