Combine shrimp and onion in a large bowl. Add capers and caper brine.

Place olive oil, vinegar, sugar, Worcestershire sauce, lemon zest, lemon juice, salt, and pepper in a medium bowl. Whisk until sugar is dissolved. Pour over shrimp. Cover and refrigerate 24 hours, stirring occasionally.

Cut tomatoes in half; add to shrimp, and toss to combine. Cover and refrigerate 12 hours.