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1
Preheat the oven to 425 and line a large, rimmed baking sheet with parchment paper.
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2
In a large bowl, toss the potatoes, parsnips, celery root, beets and garlic cloves with 3 tablespoons of the oil and season with salt.
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3
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.
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4
Let cool slightly.
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5
Meanwhile, in a medium ovenproof skillet, heat the remaining 1 tablespoon of olive oil until shimmering.
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6
Season the roast with salt.
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7
Add it to the skillet and cook over moderately high heat, turning, until browned all over, about 5 minutes.
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8
Transfer the roast to a plate and pour off the fat in the skillet.
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9
Add 2 tablespoons of water to the skillet and cook, scraping up the browned bits on the bottom of the pan.
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10
Pour the pan juices into the large bowl.
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11
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.
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12
Transfer the roast to a board; season with black pepper.
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13
Squeeze the roasted garlic cloves from their skins into the meat juices in the bowl and mash with a fork.
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14
Whisk in the vinegar, peppercorns, brine and honey.
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15
Add the roasted vegetables to the bowl and toss with the dressing.
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16
Add the arugula and toss again.
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17
Carve the roast into thin slices and serve warm or at room temperature with the salad.
