If you don’t have fiddleheads on hand, substitute asparagus that has been stemmed and cut into 1-inch pieces.
Preheat oven to 350 degrees F. Toss potatoes and carrots with garlic, coriander, ½ teaspoon salt, ½ teaspoon pepper, and 1 tablespoon of the cooking oil. Spread on a baking sheet and bake 1 hour, turning occasionally, until brown at the edges.
Meanwhile, bring a pot full of salted water to a boil. Add fiddleheads and cook until just tender, about 2 minutes. Remove with a strainer and immediately plunge into ice melts to stop cooking. Drain and set aside.
Rub the catfish on both sides with remaining salt, pepper, oil, and wine. Place in a baking dish with the wine. Turn oven to broil and place fish under broiler, about 2 inches from the heat. Broil about 8 minutes, or until fish is cooked through and springy to the touch.
While the fish is cooking, toss fiddleheads with olive oil in a skillet over medium heat, until warmed through. To serve, remove carrots and potatoes from the oven and apportion among four plates or shallow bowls. Place a piece of fish atop each potato-carrot mixture and scatter fiddleheads all around the side of each plate. Drizzle the hot juices from the baking dish over each plate, and serve immediately.