A generous amount of red onion cooks down into a jammy, caramelized spread, adding a silky, savory contrast to the quickly cooked lemony shrimp in this traditional Parsi side dish. Cilantro stems and leaves add a bold, herbal crunch.
Toss together prawns and lemon juice in a large bowl; let stand, stirring occasionally, 10 minutes. Rinse and drain well; set aside.
Heat oil in a wok or large high-sided skillet over high. Add onions, serrano chiles, and garlic; cook, stirring often, until onions are golden brown and softened, 20 to 30 minutes. Meanwhile, cut cilantro stems into 1-inch pieces; set aside. Roughly chop cilantro leaves; keep separate from stems, and set aside.
Stir ketchup, green chutney, tomato achaar, and salt into onion mixture. Cook, stirring occasionally, until mixture begins to caramelize around edges of pan, 3 to 5 minutes. Stir in prawns; cook, stirring occasionally, until prawns are opaque and almost cooked through, 3 to 5 minutes. Stir in sugar, vinegar, cilantro stems, and half of cilantro leaves. Cook, stirring often, until sugar is dissolved and cilantro is wilted, about 1 minute. Garnish with remaining cilantro leaves. Serve immediately with dhan dar.