1. Prepare cornbread according to package directions; cool completely (about 25 minutes), and crumble. Stir together dressing, mayonnaise, buttermilk, and pepper until blended.
2. Layer shredded lettuce, crumbled cornbread, and next 6 ingredients in Mason jars or a 6-qt. bowl; spoon half of dressing mixture over top. Cover and chill 3 to 24 hours. Sprinkle with green onions just before serving. Serve with remaining half of dressing mixture on the side.
Note: We tested with Martha White Buttermilk Cornbread Mix.