Preheat the oven to 180ºC. Meanwhile, start the beurre rouge reduction.
Heat a cast-iron skillet over high heat and generously cover the bottom of the skillet with canola oil. Season the trussed monkfish with salt on both sides. Once the oil starts to smoke, add the monkfish, bone side up, to the pan to sear. After about 30 seconds, reduce the heat to medium-high. After 2 minutes, once the fish is well browned, use the fish spatula to rotate the fish on its edge, balancing it against the side of the skillet. After 1 minute, rotate the fish to brown the opposite edge, again balancing it against the side of the skillet.
Turn the heat down to low and transfer the monkfish to a cooling rack set over a sheet pan. Pour the oil from the skillet into a heat-safe bowl. Return the skillet to the stove and add a fresh layer of canola oil. Add 2/3 of the butter and return the fish, bone side down, to the skillet. Add the garlic and thyme onto the top of the fish. Add the remaining butter and return the heat to medium-high. Using a plating spoon, baste the monkfish with the butter for approximately 30 seconds, then place it in the oven.
pan-roasted monkfish tail with beurre rouge
Roast the fish in the oven until the monkfish is cooked through, 12 to 15 minutes. The fish is cooked when a thermometer inserted into the thickest part of the fish registers 57°C.
Remove the fish from the oven and baste a few more times. Transfer the fish to a cooling rack set over a sheet pan, spoon the butter evenly over the top of the fish, and let it rest for 10 minutes. While the fish rests, finish the beurre rouge.
With the monkfish backbone side down, remove the twine with kitchen scissors. Place the monkfish on a paper towel on the cutting board to drain. Use a chef’s knife to carve the monkfish tail along the bone, creating two long pieces of fish. Place each piece of fish, cut side down, on a paper towel to wick away additional moisture. Trim the ends of the fish and cut each piece in half.
Season with Maldon sea salt.
Spoon beurre rouge in the center of each serving dish and top with a piece of monkfish. Garnish with a sprig of chervil.