Slice the croissant in half lengthwise.
In a small bowl, mix mayonnaise and Dijon mustard. Spread the mixture evenly on both halves of the croissant.
Layer the turkey slices on the bottom half of the croissant.
Add Havarti cheese slices on top of the turkey.
Place the lettuce and tomato slices (if using) over the cheese.
Top with the other half of the croissant and serve immediately, or wrap it up for lunch on the go.