Preheat oven to 350℉. Cut the day old croissants into ½-1" pieces and set aside.
In a medium pot, melt the butter over medium-low. Once bubbling and hot, add the onion, garlic, and celery. Cook for 5 minutes until softened. Add in the herbs and the salt and stir well, cooking for another 30-60 seconds.
Remove from heat and mix in the croissant pieces. Stir until well combined before adding to an oven safe baking dish.
Whisk the egg with the broth and pour over the stuffing. Stir well to combine then place in the preheated oven.
Bake for one hour, uncovered, stirring once or twice while baking.