Preheat the oven to 400°F (200°C). Line a baking sheet with a reusable baking mat or parchment paper.
In a small bowl, stir together the butter, miso, ginger, garlic, and sesame oil. Set aside.
Arrange the potatoes on the prepared baking sheet. Season with salt and pepper and drizzle with ½ tablespoon sunflower oil. Toss to coat.
Roast the potatoes until tender, 10-12 minutes.
While the potatoes roast, sear the sea bass. Pat each fillet dry with a clean kitchen towel or paper towel and season on both sides with salt and pepper.
Heat 2 tablespoons of sunflower oil in a medium skillet over medium-high heat. Once the oil is nearly smoking, add the sea bass, skin-side down, and cook until the skin is crispy, 3-4 minutes. Flip and sear the other side until golden brown, about 3 minutes. Remove the fish from the pan.
Remove the potatoes from the oven, leaving the oven on. Move the potatoes to a quarter of the pan. Place the fish, skin-side down, on the other quarter and spread the bok choy on the empty half. Brush the miso butter over each fish fillet, then flip and brush more on the skin side. Drizzle the remaining ½ tablespoon sunflower oil over the bok choy and season with salt and pepper. Toss to coat.
Return the baking sheet to the oven and bake until the fish is completely opaque and just cooked through, 8-10 minutes. The internal temperature should reach 145°F (63°C).
Divide the fish, potatoes, and bok choy between 2 plates. Garnish with sesame seeds and serve immediately.
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