Spray 2-quart (11-inch x 7-inch) baking dish with nonstick cooking spray. Place 6 slices French bread in single layer on bottom of baking dish. Top each bread with ham and Cheddar cheese slices. Top each with remaining slices French bread, creating a sandwich.
Whisk together, in medium bowl, eggs, milk and Dijon mustard. Pour mixture slowly over stuffed French bread. Cover; refrigerate 6 hours or overnight.
Heat oven to 350° F. Bake, uncovered, 50-55 minutes or until golden brown and knife inserted in center comes out clean.