How to Make It
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.