- Preheat oven to 425ºF (220ºC). Using spice grinder, pulse half the mushrooms to a fine powder. Set aside.
- In heatproof bowl, pour boiling water over remaining mushrooms. Soak 20 min., or until tender. Strain, reserving soaking liquid. Pat mushrooms dry and very finely chop; mix with butter, garlic, lemon zest and thyme. Refrigerate butter mixture.
- Pat roast dry with paper towel. Heat oil in large skillet over medium-high heat. Sear 10 min. until browned all over. Reserve skillet with browned bits for later. Transfer roast onto baking sheet or another oven-proof skillet. Spread mustard on roast then sprinkle with reserved mushroom powder, salt and pepper. Place roast in preheated oven 25 to 30 min., or until meat thermometer inserted in centre registers 145°F (63C) for medium-rare. Transfer roast to a cutting board, cover loosely with foil. Let rest 15 to 20 min.
- Meanwhile, heat reserved skillet over medium-high heat and stir in the wine, scraping up browned bits. In cup, whisk flour with 1/4 cup (60 mL) of the beef broth until smooth. Stir into skillet along with remaining broth and reserved mushroom soaking liquid. Gradually whisk knobs of the cold mushroom butter into skillet mixture, whisking constantly 2 to 3 min. until sauce is glossy. Remove kitchen twine from roast, if it was tied. Slice tenderloin and serve with sauce.
Tip: Use a coffee grinder to chop dried mushrooms into a powder.