1
Preheat the oven to 350°F.
2
Combine the cream, dried porcinis, brandy and 1/2 teaspoon salt in a saucepan. Bring to a simmer and cook until reduced by ⅓. Transfer to a blender and purée. Stir in the vinegar.
3
Heat a large skillet over medium heat with the butter. When melted, add the onions and mushrooms cook, reducing heat as necessary to prevent burning, until caramelized, about 10 minutes.
4
Transfer the onions and mushrooms to a baking dish, along with the heavy cream mixture, bread cubes, milk, egg yolks, sage, salt and pepper. Mix everything well.
5
Cover with tin foil and bake for 20 minutes, until set. Uncover and bake for 10 minutes more, until golden on top.