Beef Brisket with Wild Mushrooms

Ingredients

• 1 head garlic ((cloves separated but unpeeled))
• ½ ounce dried porcini mushrooms
• ⅔ cup boiling water
• 4 tablespoons olive oil ((divided))
• 1 2 1/2- to 3-pound piece beef brisket
• Kosher salt and freshly ground black pepper ((to taste))
• 1 red onion ((halved and sliced))
• 2 tablespoons all-purpose flour ((or potato starch))
• 1 cup dry red wine
• 1 ½ cups beef broth
• 1 teaspoon chopped fresh thyme leaves
• 1 pound sliced wild mushrooms ((,any sort))
• 3 tablespoons balsamic vinegar