Preheat your oven to 375°F and line a baking sheet with parchment paper.
In a medium bowl, mix together the mayonnaise and Dijon mustard. Season the shredded chicken with salt and pepper.
Butter the outside of each hoagie roll. Place them butter-side down on the baking sheet.
Spread the mayo-mustard mixture on the inside of each roll. Layer with spinach leaves, chicken, tomato slices, and top with shredded mozzarella.
Close the sandwiches and bake for 8-10 minutes until the cheese is melted and the bread is golden and crispy.
Let cool for 2 minutes before slicing in half and serving. Enjoy while hot!