1. Combine first 3 ingredients in a small bowl; stir well. Set mixture aside.
2. Heat a grill pan over medium-high heat. Place 4 bread slices in a single layer on grill pan; grill 3 minutes on one side or until grill marks appear. Remove bread from grill pan. Repeat procedure with remaining bread.
3. Coat grill pan with cooking spray. Sprinkle turkey evenly with salt. Add turkey to pan; grill 3 minutes or until done, turning after 1 1/2 minutes.
4. Spread 2 teaspoons mayonnaise mixture over untoasted side of each of 4 bread slices; top each serving with about 2 1/4 ounces turkey. Divide plum slices evenly among sandwiches; top each serving with 4 basil leaves. Top sandwiches with remaining 4 bread slices.