How to Make It
In a mini food processor, combine the parsley with the rosemary, brown sugar and 2 tablespoons of the olive oil; process to a paste. Coat the tenderloin with the paste, cover tightly with plastic wrap and refrigerate for at least 2 hours or overnight.
Bring a large saucepan of salted water to a boil. Line a large bowl with paper towels. Add the broccoli and cauliflower to the saucepan and boil until crisp-tender, about 2 minutes. Using a slotted spoon, transfer the broccoli and cauliflower to the bowl and pat dry. Discard the paper towels.
Add the potatoes to the saucepan and cook over moderately high heat until tender, about 10 minutes. Drain the potatoes and return to the saucepan. Shake the saucepan over moderately high heat for about 20 seconds to dry the potatoes. Mash the potatoes and add them to the bowl with the broccoli and cauliflower. Mix well and season with salt and pepper.
In a large nonstick skillet, heat the remaining 1 tablespoon of olive oil. Add the pancetta and cook over moderately low heat, stirring a few times, until crisp, about 5 minutes. Stir the pancetta and olive oil into the potato hash.
In the same skillet, heat 2 tablespoons of the vegetable oil. Spread the hash in the skillet in an even layer and cook over high heat until browned and crisp on the bottom, about 3 minutes. Using a spatula, flip the hash over in sections. Carefully pour the remaining 1 tablespoon of vegetable oil around the edge of the hash and cook until browned on the other side, about 3 minutes. Remove from the heat.
Light a grill. Unwrap the tenderloin and cut it crosswise into four 1 1/2-inch-thick medallions. Season with salt and pepper and grill over moderately high heat, turning once, until charred on the outside and an instant-read thermometer inserted in the thickest part of the medallion registers 125° for medium-rare, about 8 minutes. Transfer to a platter and let rest for 5 minutes.
In a small saucepan, bring the beef stock to a boil and stir in the mustard. Pour in any accumulated meat juices and season the jus with salt and pepper.
Reheat the hash until sizzling. Transfer the hash to plates and drizzle with the truffle oil, if using. Arrange the beef medallions alongside the hash. Drizzle with the jus and serve right away.