Preheat your oven to 400°F and line a baking sheet with parchment paper for easy cleanup.
In a medium mixing bowl, toss the cubed red potatoes with duck fat, chopped rosemary, thyme, sea salt, and black pepper until well coated.
Spread the potatoes in a single layer on the prepared baking sheet and roast for 25-30 minutes until they are golden and crispy on the outside.
While the potatoes are roasting, place the duck legs skin-side down in a cold cast-iron skillet, then turn the heat to medium.
Sear the duck for 8-10 minutes without flipping, allowing the fat to render out and the skin to become shatteringly crisp.
Transfer the skillet to the oven for the final 5 minutes of the potato roasting time to ensure the duck is heated through to the bone.
Steam the haricots verts with the minced garlic for 3-5 minutes until they are bright green and tender-crisp.
Plate the crispy duck leg immediately alongside the roasted herb potatoes and the garlicky greens.