Preheat the oven to 350° F. Spray 9 x 13 baking dish with cooking spray.

Place torn croissants in the baking dish and top with butter slices. Bake for 8 minutes, until the croissants are lightly toasted.

Heat the olive oil in a skillet over medium heat. Saute mushrooms for 8-10 minutes or until golden, season with salt and pepper to taste. Cook undisturbed for 5 minutes or until golden and caramelized. Add the garlic and thyme, and cook for an additional minute. Remove from the heat and set aside.

Whisk together the cream, milk, eggs, mustard, cayenne spice, and a pinch of salt and pepper in a large bowl. Add the toasted croissants, mushrooms, ham, and Brie cheese, and gently combine. Pour the mixture back into the casserole dish. Top evenly with Fontinella cheese and sprinkle with everything spice.

Cover with foil and bake for 30 minutes. Remove the foil, then continue baking for another 10-15 minutes or until the croissants are golden on top. Serve warm.