Preheat your oven to 400°F (200°C).
Slice the fingerling potatoes in half lengthwise and place them on a parchment-lined baking sheet.
Mince the garlic, rosemary, and thyme, then toss with the potatoes, duck fat, sea salt, and black pepper until evenly coated.
Roast the potatoes for 20-25 minutes, flipping halfway through, until they are golden brown and tender.
While the potatoes roast, place the duck legs skin-side down in a cold cast-iron skillet.
Turn the heat to medium and allow the fat to render out for 8-10 minutes without moving the duck, until the skin becomes very dark gold and crispy.
Flip the duck legs and cook for an additional 3 minutes on the flesh side to ensure they are heated through.
Arrange a bed of fresh arugula on a plate, top with the herb-roasted potatoes, and place the crispy duck legs alongside.