How to Make It
Grate the cut sides of the tomatoes against the large holes of a box grater set in a shallow bowl. Discard the skins. Add the scallions, chile, coriander, caraway, parsley, sugar and vinegar to the grated tomatoes. Using the flat side of a chef's knife, mash the garlic to a paste with 1/4 teaspoon of salt. Stir the paste and 3 tablespoons of the olive oil into the sauce and season with salt. Let the Kerkennaise sauce stand at room temperature for 1 hour.
Light a grill. In a medium bowl, toss the shrimp with the remaining 3 tablespoons of olive oil and season with salt and pepper. Grill the shrimp over a moderately hot fire until just cooked through, about 8 minutes. Transfer the shrimp to plates and sprinkle with the capers. Serve with the Kerkennaise sauce.