For a spicier dish, reserve and add the jalapeño seeds to the rice. This recipe requires a 12-inch nonstick skillet with a tight-fitting lid.

Process cilantro, spinach, jalapeño, and water in blender until smooth, about 2 minutes, scraping down sides of blender jar as needed. Transfer 1 tablespoon cilantro mixture to small bowl and whisk in mayonnaise, lime juice, and ⅛ teaspoon salt; set aside sauce. Reserve remaining cilantro mixture separately.

Combine chili powder and 1 teaspoon salt in small bowl. Pat cod dry with paper towels and sprinkle all over with salt mixture. Heat 1 tablespoon oil in 12-inch nonstick skillet over medium-high heat until just smoking. Place cod in skillet and cook until well browned on first side, about 2 minutes. Using 2 spatulas, carefully transfer cod to plate, browned side up. Wipe out skillet with paper towels.

Heat remaining 1 tablespoon oil in now-empty skillet over medium-high heat until shimmering. Add poblanos and ⅛ teaspoon salt and cook, stirring occasionally, until lightly browned, about 2 minutes. Stir in rice, scallion whites, and garlic and cook, stirring often, until edges of rice are translucent, about 2 minutes. Stir in broth, remaining cilantro mixture, and remaining ¾ teaspoon salt and bring to boil. Cover, reduce heat to medium-low, and simmer for 10 minutes.

Carefully place cod browned side up on top of rice mixture. Cover skillet and cook until remaining liquid is absorbed and cod flakes apart when gently prodded with paring knife and registers 135 degrees, 8 to 10 minutes. Off heat, drizzle cod with sauce and sprinkle with scallion greens. Serve with lime wedges.