Preheat the oven to 350 degrees F. Grease a 9 x 13-inch Pyrex Deep baking dish.Arrange the torn croissants in the baking dish and lay 2 tablespoons of thinly sliced butter over the croissants. Transfer to the oven and bake 5-8 minutes, until the croissants are lightly toasted.Meanwhile, heat the olive oil In a large skillet over medium heat. When the oil shimmers, add the mushrooms, season with salt and pepper. Cook undisturbed for 5 minutes or until golden, stir and continue cooking until the mushrooms have caramelized, 3-5 minutes. Add the garlic and thyme, cook an additional minute. Remove from the heat and set aside.In a large bowl, whisk together the cream, milk, eggs, mustard, cayenne, 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 Swiss cheese and sprinkle with everything spice.Cover the dish 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.