Preheat the oven to 350 degrees F. Butter a 9 x 13-inch Deep baking dish.

Arrange the torn croissants in the baking dish and scatter the 2 tablespoons of thinly sliced butter over the croissants. Transfer to the oven and bake 5-8 minutes, until the croissants are lightly toasted.

Heat the olive oil in a large skillet over medium high heat. When the oil is hot, add the mushrooms and shallots. Season with salt and pepper. Saute until golden brown, about 5 minutes. Add the garlic and thyme, cook an additional 1 minute. Remove from the heat and set aside to cool.

In a large bowl whisk together the heavy cream, eggs, white wine and a pinch of salt and pepper. Add the mushroom mixture to the cream and stir to combine. Pour the cream mixture over the toasted croissants in the casserole dish, and gently stir to combine. Arrange the torn prosciutto over the top. Top with the shredded gruyere cheese.

Cover the dish with foil and bake for 30 minutes. Remove the foil, then continue baking for another 10 minutes or until the croissants are golden on top and the cheese is bubbly. Serve!