Preparation

• Peel and chop the garlic clove. Sprinkle with 1/4 tsp. of the salt and, using the side of a chef’s knife, mash and scrape the garlic into a paste. Transfer to a medium bowl and whisk in half the onion, 2 Tbs. of the olive oil, 2 Tbs. of the parsley, 1 Tbs. of the lemon juice, 1 tsp. of the zest, and a generous 1/8 tsp. pepper. Add the shrimp and marinate, stirring occasionally, for 20 minutes.
• Meanwhile, in another bowl, stir together the tomatoes, 3/4 tsp. salt, the remaining onion, and the remaining 1/4 cup olive oil, 1/4 cup parsley, 2 Tbs. lemon juice, 2 tsp. zest, and 1/4 tsp. pepper. Stir from time to time.
• Position an oven rack 3 to 4 inches from the broiler element and heat the broiler to high. Line the bottom of a broiler pan with foil and replace the perforated top part of the pan. Arrange the shrimp on the broiler pan. Broil until the shrimp are beginning to turn bright pink and are firm to the touch on top, about 3 minutes. Turn the shrimp over, rotate the broiler pan from back to front, and broil until the shrimp are just opaque throughout (cut into a piece to check), 1 to 2 minutes longer.
• To serve, arrange the watercress on 4 plates, top with the tomatoes and their sauce, and arrange 2 shrimp on top.

Serve the salad with some grilled garlic bread to round out the meal.

You can grill the shrimp instead of broiling them.