Preheat broiler with rack 8 inches from heat source. Toss onion with lime juice and a pinch of salt. Let stand at least 10 minutes and up to 1 hour.
Meanwhile, mix scallions, jalapenos, chopped cilantro, a pinch of salt, and 2 tablespoons oil. Transfer 1/2 cup to a small bowl (set remaining mixture aside), and stir in remaining 2 teaspoons oil. Rub all over fish.
Reheat beans and their liquid. When hot, stir in 3 tablespoons reserved scallion-cilantro mixture. Remove from heat; cover.
Broil fish, flipping when scallions and cilantro begin to brown, about 6 minutes. Continue to cook until opaque throughout, about 4 minutes more.
Serve beans and fish with tortillas, remaining scallion-cilantro mixture, lime wedges, onions, cheese, radishes, and lettuce.