Preheat the oven to 350°F (175°C).
In a small bowl, mix the mayonnaise and Dijon mustard.
Spread the mixture onto the inside of each croissant.
Layer two slices of ham and one slice of Swiss cheese inside each croissant.
Close the croissants and place them in a baking dish.
Mix melted butter with poppy seeds, garlic powder, and onion powder, then brush over the croissants.
Cover the dish with foil and bake for 15 minutes.
Uncover and bake an additional 5 minutes until golden and melty.