Ingredients

• 1/4 cup vegetable oil
• 3 tablespoons yellow mustard seeds
• 1 tablespoon ground coriander
• 1/2 teaspoon ground cardamom
• 2 medium shallots, thinly sliced
• 2 garlic cloves, chopped
• 2 tablespoons chopped peeled ginger
• 1/4 cup fish sauce (such as nam pla or nuoc nam)
• 1/4 cup sugar
• 1 tablespoon ground turmeric
• 1 teaspoon crushed red pepper flakes
• 4 beefsteak tomatoes (about 2 pounds), coarsely chopped
• 2 tablespoons fresh lime juice
• Kosher salt
• 4 1-pound whole branzino, cleaned
• Kosher salt, freshly ground pepper
• 1 cup (packed) fresh tender herbs (such as basil, dill, cilantro, and/or flat-leaf parsley)

Instructions

1. Heat oil in a large saucepan over medium heat; add mustard seeds, coriander, and cardamom and cook, stirring often, until mustard seeds begin to pop, about 1 minute. Add shallots, garlic, and ginger and cook, stirring often, until softened, about 5 minutes. Add fish sauce, sugar, turmeric, and red pepper flakes and cook, stirring constantly, until sugar is dissolved, about 1 minute.
2. Add tomatoes. Bring to a boil, reduce heat, and simmer, stirring often, until tomatoes are falling apart and mixture thickens, 35-45 minutes. Mix in lime juice; season with salt and pepper. Let cool (you should have about 3 cups).
3. DO AHEAD: Tomato jam can be made 3 days ahead. Cover and chill.
4. For branzino: Preheat oven to 350°F. Season fish lightly with salt and pepper. Lay 4 large rectangular pieces of parchment paper on a work surface. Spoon 1/4 cup reserved tomato jam in the center of 1 side of each sheet; top with fish, then another 1/2 cup tomato jam. Fold parchment paper over fish and crimp edges tightly to form a sealed packet. Place packets on 2 large rimmed baking sheets. Bake fish, rotating halfway through, 30 minutes. Open packets and top fish with herbs.
5. Preheat oven to 350°F. Season fish lightly with salt and pepper. Lay 4 large rectangular pieces of parchment paper on a work surface. Spoon 1/4 cup reserved tomato jam in the center of 1 side of each sheet; top with fish, then another 1/2 cup tomato jam. Fold parchment paper over fish and crimp edges tightly to form a sealed packet. Place packets on 2 large rimmed baking sheets. Bake fish, rotating halfway through, 30 minutes.
6. Open packets and top fish with herbs.