If you've never had it before, dorade, also sold as sea bream in some markets, is a wonderful firm, white-fleshed fish that cooks quickly and has a mild flavor. A neat little trick we use to get the skin nice and crisp is to lay another, smaller sauté pan on top of the fillet as it cooks.
In a saucepan, heat the oil over medium heat. Add the onion and garlic. Sauté until the onion becomes translucent. Add the chiles de arbol and sauté for another 2 minutes.
Add the clam juice, tamarind concentrate, and sugar. Boil for 2-3 minutes and reduce the heat to low. Simmer for 10 minutes. Add the salt and remove from the heat. Pour the mixture into a blender and mix well. Strain the sauce.
Heat the oil in a skillet over high heat. Season the fillets with salt and pepper, to taste. Add the fillets skin side down. Use a smaller skillet to press down the fish with light pressure. Cook until the skin is light brown and crisp. Flip the fish and cook for 1 more minute. Serve with cilantro rice and tamarind sauce.