Preheat the broiler and line a rimmed baking sheet with foil. Toss the cabbage, red onion, 1 tablespoon plus 1 teaspoon vegetable oil and 1/2 teaspoon salt on the prepared pan. Broil, turning occasionally, until the vegetables are crisp-tender and lightly charred, 6 to 8 minutes. Transfer to a large bowl and stir in the bell pepper and 1 tablespoon lime juice; season with salt. Reserve the baking sheet.
Meanwhile, combine the mayonnaise, the remaining 1 tablespoon lime juice, the chopped cilantro, jalapenos and brine and 3/4 teaspoon chile powder in a small bowl; set aside.
Rub the fish with the remaining 2 teaspoons vegetable oil and 1/4 teaspoon chile powder; season with salt and arrange on the baking sheet. Broil the fish until just cooked through, 5 to 7 minutes.
Serve the fish with the mayonnaise sauce, slaw, cornbread and lime wedges. Top with the cilantro leaves.