Preheat your oven to 400°F (200°C).
Peel and dice the carrots and parsnips into uniform 1/2-inch cubes for even roasting.
On a rimmed baking sheet, toss the diced vegetables with the duck fat, sea salt, black pepper, and fresh thyme.
Spread the vegetables in a single layer and roast for 20-25 minutes until they are tender and caramelized.
While the vegetables are roasting, place the duck legs skin-side down in a cold cast-iron skillet.
Turn the heat to medium and sear the duck for 8-10 minutes, allowing the fat to render and the skin to become golden and crispy.
Flip the duck legs and transfer the skillet to the oven for the final 5 minutes of roasting to ensure they are heated through.
Remove the duck from the oven and let it rest for 2 minutes on a cutting board.
Toss the baby spinach with the hot roasted vegetables and balsamic vinegar directly on the baking sheet until the greens are just wilted.
Plate the vegetable medley and top with the crispy duck legs to serve.