In glass bowl, combine mangos, bell pepper, cilantro, red onion, vinegar, sugar, salt, pepper, and olive oil. (Wait to add avocado.) Chill 30 minutes.
Season fish with salt and pepper. Combine vinegar, sugar, and olive oil, and drizzle over fish. Let stand at room temperature 30 minutes. Preheat grill to medium. Grill fish 10 minutes per inch of thickness, turning once.
Just before serving, peel, seed, and dice avocado. Add to salsa mixture. Stir to blend, and adjust seasonings to your desired taste. Serve over fish.
This is excellent served over quinoa or on a bed of leafy greens, with a side salad. Enjoy with your favorite sangria or white wine!