Preheat grill to medium heat. Oil the cedar plank on one side. Assemble the plank. Toss the tomatoes and onions with 1 tablespoon of the oil and Cajun seasoning (your preferred brand).
Lay out the tomatoes and onions on the cedar plank, making sure not to get too close to the edge. Toss the shrimp with the remaining oil, and season with Cajun seasoning(your preferred brand).
Spread the shrimp on top of the onions and tomatoes. Place in the center of the grill. Cover and cook for about 10 minutes. Serve with lemon butter.