Step 1Combine mayonnaise, basil and lemon zest. Set aside.
Step 2Lightly spray zucchini with nonstick cooking spray. Grill zucchini, a portion at a time, on preheated grill pan over high heat until tender yet slightly crunchy, 2 1/2 to 3 minutes per side.
Step 3To assemble sandwich, spread 1 tablespoon mayonnaise mixture on 2 slices bread. Top 1 slice bread with 1/2 cup arugula, 3 tomato slices and 1/4 cup red onion. Lightly season with salt and pepper to taste. Top with 3 slices zucchini. Top with second bread slice. Repeat for second sandwich. Cover remaining mayonnaise and refrigerate for another use.