Preheat the oven to 350 degrees F- grease a 9 x 13-inch baking dish.
Arrange the torn croissants in the baking dish and transfer to the oven and bake 5-8 minutes, until the croissants are lightly toasted.
Meanwhile, heat the oil in large skillet over medium heat. When the oil shimmers, add the mushrooms and season. Cook undisturbed for 5 minutes or until golden, stir and continue cooking until the mushrooms have caramelized, 3-5 minutes. Remove from the heat and set aside.
In a large bowl, whisk together the cream, milk, eggs, mustard, spices and a pinch each of salt and pepper. Add the toasted croissants, mushrooms, ham, and Brie cheese, gently toss to combine. Pour the mixture back into the casserole dish. Top evenly with cheese.
Cover the dish with foil and bake for 25 minutes. Remove the foil, then continue baking for another 10-15 minutes or until the croissants are golden on top. Serve warm.
