Heat oil in a large sauté pan. Add garlic and cook until fragrant, about 1 minute.
Add shrimp and pancetta. Cook until shrimp becomes pink, about 3-5 minutes, depending on their size. Add bruschetta and lemon juice and cook until warmed through, about 2 minutes.
Meanwhile, slice polenta into ½” rounds. Place in a microwave-safe bowl and add 1 tablespoon water. Heat polenta and water in microwave for 1 minute. Mix with a fork. Heat and stir until desired consistency.
Divide polenta on plates, top with shrimp and bruschetta mixture. Sprinkle with onions and lemon zest. Salt and pepper to taste.