Cut croissants in half, lay on a baking sheet, and toast lightly in the oven. Remove and let cool.
In a bowl, mix together eggs, cream, and milk. Add dry mustard, salt, and pepper and stir in. Add gruyere and stir in.
Lay one croissant cut side up in the bottom of the ramekin. Fit the other croissant, cut-side out, around the edges of the ramekin. Pour the egg mixture over top. Place in the fridge for one to two hours.
Remove from the fridge and place ramekins on a baking tray. Bake at 325 F (160 C) for 40 minutes.
Meanwhile, in a skillet, add pancetta and cook a few minutes Add shallots and garlic and saute until soft. Add butter to the skillet and let melt. Add mushrooms and cook until starting to brown. Remove from heat. Add fresh thyme leaves and grind in pepper and stir in.
Remove ramekins from oven. Remove each croissant bake to a separate plate. Divide mushroom mixture between croissants. Drizzle truffle oil on top. Shave fresh Parmesan on top. Garnish with fresh chives.