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 ¼ 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 mustard to form a crust.
Place tenderloin in a baking dish; cover with aluminum foil. Roast 15 minutes. Remove foil; roast 10 to 20 minutes longer, checking every 5 minutes, until a thermometer in the middle of the tenderloin reaches 125°
Meanwhile, make the balsamic reduction: 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.)
Serve alongside meat.