Preheat your oven to 375°F (190°C). Lightly oil a 9 x 13-inch baking dish or coat it with nonstick spray to prevent the croissants from sticking during baking.

In a small bowl, combine the room temperature unsalted butter, Dijon mustard, honey, and thinly sliced green onions. Season the mixture with sea salt and black pepper to taste. This spread will add flavor and moisture to your croissant sandwiches. I recommend using a high-quality butter for an extra rich taste.

Slice the croissants in half horizontally if not already done. Spread the prepared mustard-butter mixture (from Step 2) evenly on the inside of each croissant half. Layer the bottom half of each croissant with smoked deli ham and diagonally sliced Swiss cheese, then place the top halves over the fillings to create 12 sandwiches.

Arrange all assembled croissant sandwiches in a single layer in the prepared baking dish from Step 1. In a small bowl, whisk together the egg white and 1 tablespoon of water to make an egg wash. Brush the tops of the croissants with the egg wash, then sprinkle them evenly with poppy seeds.

Bake the croissant sandwiches in the preheated oven for 15–18 minutes, or until the tops are golden brown and toasted. Once baked, let them stand for 5 minutes before serving to allow the cheese to set slightly. Serve warm for the best texture and flavor. For extra flair, I sometimes serve these with a little extra Dijon or honey mustard sauce on the side.