Prepare the Quick Pickle: In a small saucepan, heat the vinegar, water, and salt. Stir until steam begins to form. Pour the hot liquid over the thinly shaved cabbage, sliced red onion, and serrano peppers. Cover with plastic wrap and let sit for at least 10 minutes, or overnight for best results.
Make the Peach Mango Salsa: In a medium bowl, combine all salsa ingredients. Stir well and chill in the refrigerator until ready to serve.
Cook the Black Beans: In a large pot or Dutch oven, add in oil over medium heat. Add the chopped yellow onion, minced carrot, minced celery, minced jalapenos, and minced garlic. Sauté for 5 minutes, or until the vegetables are softened. Add the black beans and enough water or stock to cover them. Season with the bay leaf, cumin, black pepper, and garlic powder. Bring to a boil, then reduce heat to a simmer. Cook for 1 hour, stirring occasionally and lightly mashing some of the beans to create a creamy texture. Finish with an additional tablespoon of butter, if desired.
Sear the Mahi Mahi: While the beans are simmering, combine the paprika, black pepper, garlic powder, cayenne pepper, and salt in a small bowl. Pat the Mahi Mahi fillets dry with paper towels. Season both sides of the fillets generously with the spice mixture. Heat a preheated skillet over medium-high heat. Sear the fish for 4 minutes on each side, or until golden brown and cooked through. Add a knob of butter to the pan during the last few minutes of cooking and baste the fish.
Assemble and Serve: To plate, spoon a generous portion of the black beans onto the bottom of each serving plate. Top with a seared Mahi Mahi fillet. Add a scoop of the peach mango salsa and a spoonful of the crunchy quick-pickled vegetables. Drizzle a little of the vinegar brine from the quick pickle over the dish for added flavor.