Begin by rinsing the arctic char fillets under cold water, then gently pat them dry with paper towels. Arrange the fillets in a single layer within a deep glass baking dish, skin-side down.

In a separate mixing bowl, whisk together the Dijon mustard, honey, olive oil, minced garlic, thyme, lemon juice, sea salt, and black pepper until the marinade is smooth and well blended.

Generously coat each fillet with the marinade, making sure both sides are evenly covered. Use the back of a spoon or a brush to work the mixture into the surface of the fish. Cover the dish tightly with plastic wrap and refrigerate for 30 to 40 minutes to allow the flavors to penetrate. Discard any remaining marinade once the fish has finished marinating.

Meanwhile, preheat the grill to medium-high heat. Just before placing the fish on the grates, oil them well to prevent sticking. Use folded paper towels dipped in a high smoke point oil like grapeseed or avocado oil, held with tongs, to brush the grates thoroughly.

Place the fillets onto the grill skin-side down. Let them cook undisturbed for about 5 minutes to allow the skin to crisp and the fish to set.

Carefully flip each fillet using two heatproof spatulas—one to lift and the other to support the turn. Continue grilling for another 3 to 5 minutes, or until the fish flakes easily with a fork, appears opaque, and reaches an internal temperature of 145°F (63°C).

Remove the fish gently from the grill and transfer to serving plates. Serve immediately for the best flavor and texture.