Prep: Preheat the oven to 375 degrees F. Cut croissants in half, lengthwise, to create a top and bottom for making a sandwich.

Fill Croissants: Place all of the croissant bottoms on a large rimmed baking sheet. Use 1 tablespoon of softened butter to butter the bottom halves. Place 3 slices of ham on each croissant and then 3 slices of gruyere cheese on top of the ham. Use another tablespoon of softened butter to butter the croissant tops and place them on top of the cheese.

Brush + Bake: Melt the remaining 2 tablespoons of butter and brush some on top of the croissants; reserve the remaining melted butter for later. Cover the entire pan with aluminum foil and pinch around the edges to secure. Bake in the preheated oven for 15-20 minutes or until cheese is melted.

Make Sauce: While the croissants bake, put all of the Sauce ingredients in a small bowl and whisk to fully combine. Set aside.

Finish: Remove the pan from the oven, remove the foil, and brush the croissants with the reserved melted butter. If using poppy seeds, sprinkle them on top at this time. Return the sheet pan to the oven to bake, uncovered, for 2-3 minutes.

Serve: Enjoy these croissants with the honey mustard dipping sauce!