• 1 medium red onion, thinly sliced
• 1½ cups red wine vinegar
• ¼ cup extra-virgin olive oil
• 1½ tsp. ancho chile powder
• ¼ cup lightly packed fresh cilantro leaves, chopped, plus more for garnish
• 1 jalapeño, stemmed, seeded if desired, and finely chopped
• 1 lb. flaky white fish fillets (such as mahi-mahi or cod), cut into 4 pieces
• Crema mexicana, fresh salsa, hot sauce, and lime wedges for garnish