Thread a toothpick through the tail meat and the other end of the shrimp, forming each shrimp into the shape of the number 6. Fill a medium bowl with ice water.
Bring a medium pot of generously salted water to a boil. Squeeze the juice from the lemon half into the water. Add the shrimp, and cook until pink and just cooked through, 3 to 4 minutes. Transfer the shrimp to the ice water to cool for a few minutes. Remove the toothpicks, and refrigerate the shrimp until ready to serve. (The cooked shrimp can be refrigerated for up to 1 day.)
Meanwhile, swirl the butter, oil and coriander, cumin and fennel seeds in a small saucepan over medium heat until the spices are slightly darkened and fragrant, about 1 minute. Add the tomatoes, 3 tablespoons water, tomato paste, vinegar, ginger, 1/2 teaspoon salt, sugar, cayenne pepper and curry powder, and bring to a simmer. Reduce to a low simmer, and cook, stirring occasionally and mashing the tomatoes slightly, until thick, 10 to 12 minutes. Let the chutney cool completely. (The chutney can be refrigerated up to 2 days.)
Dry the shrimp, and arrange them on a serving platter. Put a little mound of chutney on top of each (not on the tail), and garnish with a cilantro leaf.
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