How to use puff pastry with a loin tenderloin beef roast

I've never heard of putting mashed potato in this. In my opinion, the liquid content in the potato would create a soggy pastry. Just sear the tenderloin, wrap I it int the pastry and bake at 375. Put the potato on the side

You should sear the loin first, go easy with the mashed potato (maybe 1" is enough) then wrap . You should really do it upside down, that is, lay out the pastry, put down a layer of potato same length and width as the loin, put on the seared loin, then pull the pastry up and around. Trim excess, flip over and put in a hot oven. Watch like a hawk and use you instant read thermometer periodically to check the doneness of the meat. At the last minute you can brush with an egg wash pop back in the oven for a couple minutes for a nice color. Serve with horseradish .

Yes that sounds much too low, it sounds sort of like beef wellington which is usually baked in a hot oven 400 or 420 degree's depending on the size and degree of doneness about 20-35 minutes.