Preheat your oven to 400°F (200°C).
Toss the sliced carrots, parsnips, and halved brussels sprouts on a large baking sheet with the duck fat, sea salt, and black pepper.
Roast the vegetables for 20-25 minutes, tossing halfway through, until they are tender and caramelized.
While the vegetables roast, place the duck leg skin-side down in a cold cast-iron skillet.
Turn the heat to medium and cook for 6-8 minutes without moving it, allowing the skin to become deeply golden and crispy.
Flip the duck leg and cook for an additional 2-3 minutes until heated through.
In a small bowl, combine the chopped parsley, lemon zest, and minced garlic to create the gremolata.
Serve the crispy duck leg over the roasted root vegetables and garnish with the fresh gremolata.