Make the Green Rice: Combine the water and rice in saucepan with a tight-fitting lid. Season the water with salt. Bring to a boil, lower the heat, and simmer, covered, until tender, about 20 minutes. Set the rice aside, covered, for 10 minutes. Fluff the rice with a fork and stir in the chimichurri. Set aside, covered, until ready to use.
Meanwhile, make the Black Beans: Heat the oil in the saucepan over medium-high heat. Add the cumin seeds and cook, stirring, until fragrant, about 30 seconds. Add the bell pepper and onion and cook, stirring, until soft, about 8 minutes. Stir in the black beans and sugar. Bring to boil, lower the heat, and simmer for 5 minutes. Add the vinegar and season with salt and pepper. Set aside, covered, until ready to use.
Make the Salsa: Preheat a grill pan over high heat. Grill the pineapple rings, turning once, until charred, about 2 minutes. Transfer the rings to a cutting board and chop.
In a bowl, combine the pineapple , mango, bell pepper, onion, cilantro, olive oil, ginger, jalapeno, and lime juice. Season with salt and pepper and set aside. (The salsa can be prepared up to 2 days in advance.)
Make the mahi mahi: Arrange the fillets in baking pan, season with salt and pepper, and rub with the zest. Preheat a cast iron skillet over medium heat for 5 minutes. Raise the heat to high and heat the butter and oil. Working in batches, if necessary, sear the halibut fillets , turning once, until well browned and just cooked through, about 3 minutes per side.
Divide the rice and beans among the center of 6 plates, top with the fillets, and spoon some the salsa over each.
