Red Snapper Soup With Fresh Bean Sprouts And Garlic Oil

Name: Red Snapper Soup With Fresh Bean Sprouts And Garlic Oil
Ingredients: 3 quarts cold water; Two 2-pound red snappers, filleted and skinned, heads and bones reserved; 1/4 cup plus 1 tablespoon Asian fish sauce; 2 stalks of lemongrass, cut into 5-inch lengths and lightly crushed; One 1 1/4-inch piece of fresh ginger, sliced into 1/4-inch-thick rounds; 1 small onion, quartered; Kosher salt; 1/2 cup canola oil; 3 large garlic cloves, minced; 1/2 pound white mushrooms, thinly sliced; 6 ounces mung bean sprouts; 4 scallions, thinly sliced; Thinly sliced serrano chiles and lime wedges, for serving