Thaw fish, if frozen; rinse and pat dry with paper towels. Halve each fillet horizontally. In a 2-quart square baking dish place half of the fish, leaving no space between the pieces. Sprinkle with 1/4 teaspoon of the salt. Thinly slice 1 of the lemons. Lay the lemon slices and 1 cup cilantro leaves on the fish in baking dish. Top with remaining fish. Sprinkle with 1/4 teaspoon of the salt. Add 2 tablespoons water to the bottom of the dish. Cover dish with waxed paper.
Microcook on 100 percent power (high) for 3-1/2 to 4-1/2 minutes or until fish flakes easily when tested with a fork, turning dish halfway through cooking if necessary. Cool slightly. Transfer fish to a serving platter. Cover with plastic wrap and chill 2 to 4 hours.
In a medium bowl combine tomatoes, onions, and remaining 1/2 teaspoon salt. Toss gently to combine. Cover and chill while fish chills.
To serve, spoon tomato mixture next to fish. Cut remaining lemon in half; squeeze over fish and tomato mixture. Sprinkle with snipped fresh cilantro. Makes 8 side-dish servings.