Preheat the oven to 350°F. Grease a 9x13-inch baking dish.
Slice the croissants in half lengthwise and layer half of them in the bottom of the prepared baking dish.
Top with half of the sliced ham and half of the Swiss cheese.
Repeat with the remaining croissants, ham, and cheese.
In a large bowl, whisk together the eggs, milk, Dijon mustard, garlic powder, onion powder, black pepper, and salt until well combined.
Pour the egg mixture evenly over the layered croissants, ham, and cheese.
Press down gently on the croissants to ensure they soak up the egg mixture.
If desired, sprinkle chopped parsley on top for added flavor and color.
Cover the baking dish with aluminum foil and bake for 25 minutes.
Remove the foil and bake for an additional 15-20 minutes, or until the top is golden brown and the egg is set.
Let cool for 5 minutes before slicing and serving.