Make the miso marinade. Season cod fillets on all sides with 1 tsp Kosher salt and ¼ tsp black pepper. Place in a large tupperware or Ziploc bag. In a large bowl, combine ¼ cup miso paste, 2 Tbsp sake, 2 Tbsp mirin, 1 Tbsp soy sauce, 2 tsp ginger paste, 1 tsp sesame oil, and 1 tsp honey. Whisk to combine, then pour over fish. Seal container, shake, then refrigerate for at least 1 hour or up to overnight.

Cook the vegetables. Preheat oven to 400F. If needed, cut cod fillets so they fit easily in the skillet, reserving marinating liquid. Heat 2 Tbsp canola oil in a large skillet over medium-high heat. When hot, add 2 cups sliced carrots and cook for 4-5 minutes, or until beginning to soften. Add 4oz mushrooms and 1 Tbsp butter, then cook an additional 3-4 minutes, stirring occasionally.

Add scallions. Trim the ends off 2 scallions, then cut into 2" pieces. Add to the skillet and sauté until blistered, another 3-4 minutes. Season with remaining ¼ tsp Kosher salt, stir, then transfer to a bowl.

Cook the cod. Carefully add cod fillets to the warm, preheated pan. Cook for 2 minutes, then gently use a fish spatula to flip the fish over, being careful not to break it. Return cooked vegetables to the pan, then pour reserved miso marinade on top. Transfer the skillet to preheated oven and cook for 8 minutes, or until an internal temperature of 140-145F is reached.

Garnish, then serve. Thinly slice remaining scallion (discard the root), then sprinkle on miso cod along with a generous sprinkle of sesame seeds, cilantro, and scallions for garnish. Serve immediately with white rice and roasted or steamed veggies.