Preheat the oven to 350°F.
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.
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.
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.
Cover with tin foil and bake for 20 minutes, until set. Uncover and bake for 10 minutes more, until golden on top.