1 cup (packed) fresh tender herbs (such as basil, dill, cilantro, and/or flat-leaf parsley)
Directions
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.
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).
DO AHEAD: Tomato jam can be made 3 days ahead. Cover and chill.
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.
