Preheat the broiler. Heat 1 tablespoon olive oil in a medium saucepan over medium heat. Add the chopped onion and cook until softened, 3 minutes. Add the beans, salsa and 1/2 cup water. Cook over low heat, stirring occasionally, until thickened, about 8 minutes; season with salt.
Meanwhile, toss the bell peppers and sliced onions with 2 tablespoons olive oil, 1 teaspoon cumin, 1 teaspoon chili powder and 1/2 teaspoon salt on a rimmed baking sheet. Broil, without stirring, until the vegetables are browned around the edges, 4 to 6 minutes.
Toss the fish with the remaining 1 tablespoon olive oil and 1/2 teaspoon each cumin and chili powder; season generously with salt. Scatter over the roasted vegetables and broil until the fish is just cooked through, 3 to 4 minutes.
Stir the cilantro into the beans and divide among plates. Serve with the fish, roasted vegetables, tortillas, guacamole and lime wedges.