Preheat oven to 400°F. Line a large rimmed baking sheet with foil. Brush 1 tablespoon of the peanut oil over foil. Place fish on baking sheet. Brush exterior and inside cavity of fish with remaining 2 tablespoons peanut oil; drizzle 1 tablespoon of the soy sauce inside cavity. Cut 3 2-inch slits all the way to the bone on both sides of fish.
Cut 5 of the scallions diagonally into 2-inch pieces. Stuff cavity of fish with ginger, garlic, scallion pieces and two-thirds of the cilantro.
Bake fish until opaque and flakes easily with a fork, about 45 minutes. (To test for doneness, insert a small knife into slits; fish should pull away easily from the bone.) Remove fish from oven. Increase oven temperature to broil with oven rack 6 inches from heat.
Stir together sesame oil, mirin and remaining 1/4 cup soy sauce in a small bowl. Stir in crushed red pepper, if desired. Brush fish with 2 tablespoons of the soy sauce mixture; return fish to oven and broil until lightly charred in spots, about 6 minutes. Transfer to a serving platter.
Thinly slice remaining 2 scallions; remove and reserve leaves from remaining cilantro. Sprinkle sliced scallions and cilantro leaves over fish. Serve with remaining soy sauce mixture and lime wedges.