Heat butter in a large skillet over medium heat. Add chopped mushrooms and minced shallot.
Cook until mushrooms and shallot are softened and browned, about 8-10 minutes.
Add minced garlic and fresh thyme. Cook for 1 minute until fragrant.
Remove from heat and let mushroom mixture cool slightly.
In a medium bowl, combine cream cheese, Parmesan cheese, truffle oil, salt, and pepper.
Add cooled mushroom mixture and stir to combine.
Slice croissants horizontally, creating a pocket for filling.
Fill each croissant with mushroom mixture.
Brush tops of croissants with beaten egg.
Preheat oven to 375°F (190°C). Bake for 12-15 minutes until golden brown.
Cool slightly before serving.