Steamed Sea Bass with Tamarind Sauce

Name: Steamed Sea Bass with Tamarind Sauce
Ingredients: One 2 1/2-inch block of tamarind pulp (5 ounces); 3 1/4 cups water; 2 tablespoons cumin seeds; 1 tablespoon coriander seeds; 1/4 cup unsweetened coconut milk; 1/4 cup unsalted dry-roasted peanuts; 3 tablespoons minced peeled ginger; 2 teaspoons harissa; 4 tablespoons plus 1 teaspoon unsalted butter; 3 pounds nonoily white fish bones, rinsed and cut into large pieces; 5 scallions, thinly sliced; 1 garlic clove, crushed; 24 pearl onions, blanched and peeled; Four 6-ounce sea bass fillets; Salt and freshly ground pepper; 1 teaspoon fresh lime juice; 2 tablespoons finely shredded mint leaves