Preheat your oven to 400°F (200°C).
Peel and chop the carrots and parsnips into 1-inch chunks.
On a rimmed baking sheet, toss the carrots, parsnips, and smashed garlic cloves 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 vegetables roast, place the duck leg confit in a cold cast-iron skillet, skin-side down.
Turn the heat to medium and cook for 8-10 minutes without moving it, allowing the fat to render and the skin to become deeply golden and crispy.
Flip the duck leg and transfer the skillet to the oven for the final 5 minutes of the vegetable roasting time to heat through.
Plate the crispy duck over the roasted root vegetables and garnish with fresh thyme leaves.