In a mortar and pestle or a coffee grinder reserved for grinding spices, grind the peppercorns, cumin seeds, and salt until fine. Transfer the spices to a small bowl and add the lime juice. Prick the fish all over with a fork and rub the seasoning paste well into the skin. Set the fish in an ovenproof dish and leave to marinate at room temperature for about I hour, loosely covered.
Preheat the oven to 300 degrees. In a mortar and pestle or a blender, combine the garlic, green pepper, green onions, oregano, vinegar, green tomatoes, water and salt and blend to a smooth sauce. Lift up the fish in their dish and scatter 2 tablespoons of the olive oil underneath each-1- Drizzle the remaining oil over the top of the fish and rub it in. Spoon the salsa verde over the top. Cover the dish and cook the fish in the oven for 20 minutes, then carefully turn it over and baste with the sauce. Cover again and cook for 15 to 20 minutes more. Serve warm.
If using striped bass, it will take from 1 to 1 1/2 hours total to cook. Baste every 15 minutes.