Mix tomatoes, onions, cilantro, serrano chiles, extra-virgin olive oil, lime juice, avocados together in a large bowl. Adjust seasonings with salt and freshly ground black pepper. Let the flavors come together at room temperature for about 1 hour.
Prepare grill and when coals are ready, coat fish fillets with olive oil, salt and fresh pepper and grill about 4 minutes per side (depending on thickness). Plate fish and top with salsa. Serve with grilled vegetables of your choice and rice.