While the fish is in the oven, prepare the slaw. In a large bowl, whisk together the mayonnaise, lime juice, sugar, salt, garlic powder, and black pepper. Add the cabbage, green onions, cilantro (or parsley), and jalapeño pepper (if using). Toss to coat the vegetables in the dressing. Taste and season with extra salt and pepper, if necessary. Let the slaw sit until ready to serve.