1. Spread both halves of each croissant with mustard.
2. Place the bottom halves of each croissant in a buttered baking dish.
3. Add a slice of ham to each croissant bottom, and place brie on top. Place another slice of ham on top or fold larger slices of ham over the brie to make an envelope.
4. Place croissant tops on top of the ham.
5. In a large bowl, whisk together the eggs and milk.
6. Season the egg mixture with salt, pepper, hot sauce, and tarragon, then continue to whisk until incorporated.
7. Stir 1 ½ cups shredded Swiss into the egg mixture.
8. Pour egg mixture over croissants in baking dish. Press down once more on croissants so they absorb more egg mixture.
9. Allow strata to rest for at least half an hour or keep it in your fridge for up to 12 before baking. Sprinkle remaining ½ cup Swiss over strata right before baking.
10. Bake at 350˚ for 35-50 minutes, until the egg is puffed, set, and the cheese is melted.