
Preheat the oven to 350°F. Scrub the celeriac well, place it on a baking sheet, and roast until a skewer slides easily into the center, about 60 minutes. Let it cool slightly, trim away any tough skin or root ends, and dice the flesh into pieces about the same size as the shrimp.


Bring 6 cups of water to a boil in a large skillet or wide pot. Add the salt and pasta, then cook for about 7 minutes, or until the pasta is al dente. Lift the pasta out with tongs or a slotted spoon and keep it warm, reserving the cooking water in the pan.


Add the shrimp and peeled garlic cloves to the reserved pasta water. Simmer for about 3 minutes, until the shrimp turn pink and are just cooked through. Remove the shrimp and keep them warm; leave the garlic and cooking liquid in the pan.


Add the frozen spinach to the same cooking liquid and simmer just until hot and bright green. Transfer 1 cup of the hot cooking liquid, the cooked garlic, cashews, and olive oil to a food processor or blender. Blend until smooth, then add the spinach and blend again to make a creamy green sauce.


Toss the warm pasta with the spinach-cashew sauce, adding a splash more cooking liquid if needed to loosen it. Season with freshly crushed pepper to taste.


Top the pasta with the cooked shrimp and diced roasted celeriac. Finish with grated Parmesan cheese and serve warm.