In a bowl whisk together mayonnaise, sour cream, Dijon, and black pepper, and set aside.
Cut each croissant in half and toast in the toaster on low heat.
Generously spread the Dijon mayonnaise on one half of the croissant, add 3 slices of turkey breast, half slice of Havarti cheese, and a small piece of romaine lettuce. Then top with the other half of the croissant.
If you can't find Havarti cheese in the grocery store you can substitute it with Muenster cheese.