Step 1

Heat the oven to 475°F with a rack in the middle position. Line a rimmed baking sheet with foil. In a small bowl, stir together half of the hazelnuts, the coriander, garlic, zest and ¾ teaspoon each salt and pepper.

Step 2

Using your hands and 1 tablespoon of butter per cabbage wedge, rub the butter on all sides and into the layers of the cabbage. Sprinkle the wedges evenly with the spice mixture, rubbing it in to adhere; reserve the bowl. Place the cabbage wedges cut-side down on the prepared baking sheet. Cover tightly with foil and roast until a skewer inserted at the thickest part of the cabbage meets a little resistance, 20 to 30 minutes.

Step 3

Uncover the baking sheet and roast until the cabbage is deeply browned on all sides, another 15 minutes, flipping the wedges with a wide metal spatula about halfway through. Meanwhile, in the reserved bowl, whisk together the grapefruit juice, mustard, honey, ½ teaspoon salt and ¼ teaspoon pepper; set aside.

Step 4

Transfer the cabbage to a cutting board, then trim out and discard the core from each wedge. Place the wedges on a platter and drizzle each wedge with 1 tablespoon of the sauce. Sprinkle with the remaining hazelnuts and the tarragon. Serve with the remaining sauce on the side.