In a bowl whisk together mayonnaise, sour cream, Dijon, and black pepper, then set aside.
Cut each croissant in half and toast in the toaster on low heat.
Generously spread the Dijon mayonnaise on both halves of the croissant. Now place the bottom croissant half (cut side up) and add 3-5 slices of turkey breast, half a slice of Havarti cheese, (or as much as you want) and a small piece of romaine lettuce. Then top with the other half of the croissant.
Toast the croissant- Doing this creates a barrier to the moist lettuce and sauce so that the sandwich doesn't get soggy too quickly.
Don't overload the sandwich- Adding too many ingredients to your sandwich will cause the delicate croissant to tear, as well as overwhelm its yummy flavor.
Use the best parts of the romaine head- You want to have some crunch so choose bright green leaves that have stiff ribbing throughout for the best texture.
Make extra Dijon mayo- This secret sauce goes great on tons of sandwiches besides this Turkey Croissant Sandwich. Double your batch and store it in the fridge for easy use.
Cheese substitute- If you can't find Havarti cheese in the grocery store you can substitute it with Muenster cheese.