Slice the tenderloin open lengthwise, being careful not to cut through to the other side. (You want to split the meat into one large, flat piece.) Spread the mustard over both sides of the meat and rub in the pepper, pressing gently so it adheres well. Cut the meat across into 4 even protions.
In a large skillet, heat the oil over medium heat until hot. Put the tenderloin in the hot pan and cook, turning once, until an instant-read thermometer inserted in the thickest part registers 155ºF, about 10 minutes total. Transfer the meat to a plate and tent with aluminum foil to keep it warm.
Add the broth and wine to the pan and cook over medium-high heat, scraping up any browned bits that have stuck to the bottom. Continue to cook until the sauce has reduced to about 1/2 cup, 8 to 10 minutes.
Pour the sauce over the meat, season with salt and pepper and serve.
Place th e potatoes in a steamer basket fitted over a large pot of boiling water. OH MY GAWD!!! CUT THEM IN TO SMALL CHUNKS FIRST!!! Then cover and steam until the potatoes are knife-tender, 12 to 15 minutes.
While the potatoes are cooking, combine the buttermilk and regular milk in a small saucepan and cook over the lowest possible heat until just warm. Be careful not to let it boil or the milk will curdle.
Drain the potatoes, return them to their pot, add the warmed milk, and mash together to the consitency you prefer. Stir in the Parmesan and season to taste.