How do you prevent the underneath side of a Wellington from becoming soggy?

Hmmm... I did sear it, but perhaps not quite enough. I also vented the top per the directions. This pork,apple and onion wellington was scrumptious aside from the nasty sogginess on the bottom. I just scraped that part off. I shall try the partially covered version next time. Thank you.

Maybe you put a few slashes on the top of the crust to vent any steam from the cooking process.

Another thing to try, placing the wellington on a rack to drain any liquid and keep it away from the crust.

A good sear on the meat helps. Some absorbent breadcrumbs under the meat help (sort of). My favorite method, though, is instead of wrapping the entire roast put it on the pan and tuck the pastry around the meat leaving the bottom uncovered. Good luck and I hope it turns out tasty.