Make the Vegetable Relish: In a mixing bowl, combine the diced mango, diced zucchini, chopped red onion, chopped grilled (or roasted) red bell pepper, and chopped arugula. Stir until evenly mixed, then set aside while you cook the fish.
Season the Fish: Pat the fish fillets dry. Rub both sides with olive oil, then season with kosher salt and black pepper.
Grill the Fish: Heat a grill pan or grill over medium-high heat. Cook the fish until grill marks form and it flakes easily, about 3–4 minutes per side depending on thickness.
Mix the Rice and Beans: In a large bowl, combine the cooked rice and white beans. Add the diced tomatoes and chopped basil, then stir to combine.
Dress the Rice Mixture: Add the minced garlic and pour in the lemon vinaigrette. Stir until everything is evenly coated and glossy. Taste and stir again if needed so the vinaigrette is spread through the rice.
Plate and Serve: Spoon the rice-and-bean mixture onto plates, top with a fish fillet, and finish with a generous scoop of the vegetable relish. Serve right away for the best contrast between warm fish and crisp relish.