1/4 teaspoon plus 2 teaspoons kosher or sea salt, divided
1 whole beef tenderloin, about 1-3/4 pounds
2 tablespoons olive oil
1 teaspoon freshly ground black pepper.
2 tablespoons whole-grain mustard
Preheat oven to 450°. In a large skillet over low heat, toast walnuts, stirring occasionally, until golden brown and fragrant, 8-10 minutes.
Put walnuts in a food processor with rosemary, garlic, and 1/4 teaspoon salt; pulse until mixture looks like coarse sand.
Using kitchen twine, tie up meat at several points along the tenderloin to maintain uniform thickness. Season with remaining 2 teaspoons salt and pepper.
Heat a large skillet on medium-high setting. Add olive oil to pan; sear beef until browned on all sides, 4-5 minutes per side. Remove twine.
Spread a thin layer of mustard over meat. Take liberal amounts of the walnut mixture and press it firmly against mustard to form a crust.
Place tenderloin in a baking dish; cover with aluminum foil. Roast 15 minutes. Remove foil; roast 10-20 minutes longer, checking every 5 minutes, until a thermometer in the middle of the tenderloin reaches 125°.
For the sauce:
2-1/2 cups inexpensive balsamic vinegar
1/2 cup honey
1/2 cup chopped dried fruit (cranberries, apricots, golden raisins, etc.)
Combine vinegar, honey, and dried fruit in a medium-size saucepan over medium-high heat. Bring to a simmer; then reduce heat to medium-low and simmer 30-40 minutes until mixture is thick enough to lightly coat the back of a spoon. (Note that cooking vinegar can create a strong odor; we recommend good ventilation.)