Preheat your oven to 350°F (175°C).
Slice each croissant in half horizontally.
In a small bowl, mix together the mayonnaise and Dijon mustard. Spread the mixture evenly on the cut sides of each croissant.
Layer the bottom half of each croissant with sliced turkey, followed by Havarti cheese, baby spinach, and tomato slices.
Season with salt and pepper to taste.
Place the top half of the croissant on each sandwich.
Arrange the sandwiches on a baking sheet and bake in the preheated oven for about 5-7 minutes, or until the cheese is melted and the croissants are warm.
Remove from the oven, slice in half if desired, and serve immediately.