Beef Tenderloin is marinated with Asian seasonings just for flavour, reserving the marinade to use in the flavourful Finishing Sauce. Although relatively easy to make, there are quite a few components to this recipe. Simplify the dish by making the Barley and Mushrooms ahead warming them just before serving.
MARINADE AND BEEF: In bowl, whisk together oyster sauce, vinegar, sugar, fish sauce, miso paste and garlic. Place beef in sealable freezer bag; add marinade, seal and refrigerate for 2 hours or overnight. Remove beef from marinade, refrigerate remaining marinade. Pat beef dry and season with salt and pepper. Insert meat thermometer lengthwise into the center of the roast so shaft is not visible. Place on rack in shallow pan. Oven-sear roast, in 450°F (230°C) oven for 10 minutes; reduce heat to 275°F and roast, uncovered until thermometer reads 145°F (63°C) for medium-rare, about 1 hour. Meanwhile, make Mushroom Barley and Sautéed Mushrooms. When roast is cooked, transfer to cutting board, tent with foil for 15 minutes before carving. Meanwhile, make Sauce with reserved pan drippings.
MUSHROOM BARLEY : In medium saucepan, combine barley and beef stock. Bring to boil over medium-high heat; reduce heat to low and let simmer, uncovered, until barley is tender, about 30 minutes. Meanwhile, while barley is cooking, combine dried mushrooms with 1 cup of boiling water. Let sit for 10 minutes. Drain, reserving liquid for Finishing Sauce; coarsely chop soaked mushrooms. Combine chopped soaked mushrooms, barley and parsley; keep warm. (Make-ahead: Cover and refrigerate for up to 3 days. Just rewarm in saucepan over medium-low heat or microwave before serving.)
SAUTEED MUSHROOMS: Cook mushrooms in two batches, heating 1 tbsp each butter and olive oil in large skillet over medium-high heat; add half the garlic and half of the mushrooms, salt and pepper. Cook, stirring, until mushrooms are golden brown, about 12 minutes; remove from pan and set aside. Cook the second batch with 1 tbsp EACH butter and olive oil and remaining garlic and mushrooms. Add reserved mushrooms back to pan; stir in parsley. (Make-ahead: Cover and refrigerate for up to 3 days. Just rewarm in skillet over medium heat before serving.)
FINISHING SAUCE: Heat reserved pan drippings over medium-high heat; add reserved mushroom soaking liquid, reserved marinade and beef stock. Cook mixture over high heat until reduced by half, stirring to loosen any browned bits from bottom of pan, about 5 minutes. Stir in remaining 2 tbsp butter. To serve, thinly slice beef and serve with Mushroom Barley and Mixed Mushrooms; top with Finishing Sauce.