How to Make It
Preheat the oven to 425° and line a large, rimmed baking sheet with parchment paper. In a large bowl, toss the potatoes, parsnips, celery root, beets and garlic cloves with 3 tablespoons of the oil and season with salt. Spread the vegetables on the baking sheet and roast on the bottom rack of the oven for about 1 hour, stirring occasionally, until browned and very tender. Let cool slightly.
Meanwhile, in a medium ovenproof skillet, heat the remaining 1 tablespoon of olive oil until shimmering. Season the roast with salt. Add it to the skillet and cook over moderately high heat, turning, until browned all over, about 5 minutes. Transfer the roast to a plate and pour off the fat in the skillet. Add 2 tablespoons of water to the skillet and cook, scraping up the browned bits on the bottom of the pan. Pour the pan juices into the large bowl.
Return the meat to the skillet and roast in the oven for 25 minutes, until an instant-read thermometer inserted in the thickest part registers 115° for medium-rare meat. Transfer the roast to a board; season with black pepper.
Squeeze the roasted garlic cloves from their skins into the meat juices in the bowl and mash with a fork. Whisk in the vinegar, peppercorns, brine and honey. Add the roasted vegetables to the bowl and toss with the dressing. Add the arugula and toss again.
Carve the roast into thin slices and serve warm or at room temperature with the salad.
One Serving 407 cal, 13 gm fat, 2.6 gm sat fat, 43 gm carb, 9 gm fiber, 30 gm protein.