1/2 cup low-sodium soy sauce 2 tablespoons mirin 2 teaspoon toasted sesame oil 2 tablespoons packed light brown sugar 8 large garlic cloves (minced) 3 tablespoons minced peeled fresh ginger 8 scallions (white and green parts - chopped) 1 medium yellow onion (grated) 1 medium asian pear (grated) 4 1/2 beef tenderloin roast (tied)
In a large bowl or baking dish, combine the soy, mirin, sesame oil, brown sugar, garlic, ginger, scallions, onion and Asian pear, and stir to combine. Add the beef and turn to coat. Cover and marinate in the refrigerator for at least 4 hours, preferably overnight.
Put the ground Szechuen peppercorns on a plate. Remove the beef form the marinade, scraping off all the solids, and roll it in the peppercorns, pressing down to that they stick.
2 tablespoons extra-virgin olive oil
Heat a large, ovenproof skillet over medium-high heat for 5 minutes, then add the olive oil. Put the beef in the pan and sear on all sides until well browned, about 10 minutes. Then, transfer the roast to the oven and bake until an instant-read thermometer inserted into the thickest part of the roast reads 130F, about 15 minutes.
Remove from the oven and tent with foil for 15 minutes for perfect medium-rare, 20 minutes for medium. Slice and serve.