Preheat your oven to 350°F (175°C) and prepare a baking sheet lined with parchment paper.

Slice each croissant in half horizontally and place the halves cut side up on the prepared baking sheet.

In a small bowl, combine mayonnaise and stoneground mustard. Chop fresh chives and mix in if using. Spread the mixture evenly on croissant halves.

Layer deli ham and Muenster cheese on each croissant half, pressing gently.

Close the croissant halves and arrange seam side down on the baking sheet.

Bake for 10-15 minutes until cheese is melted and croissants are golden brown.

Remove from oven, let cool for a minute, then serve warm.