Brisket with Mushroom-and-Onion Gravy

Flat brisket can be of different sizes. If you end up with a large oblong piece, trim the thin end if necessary so that it fits in the roaster and cooks evenly.
Mix the first 4 ingredients together. Rub the mixture on both sides of the brisket. Place the brisket in a jelly dish lined with parchment paper; cover and refrigerate for 8 hours.Let the brisket sit at room temperature for 30 minutes. Preheat the oven to 325 degrees. Soak dried porcini mushrooms in 2 cups of boiling water for 10 minutes.Meanwhile, saute the brisket in preheated oil in a large roasting pan for 4 minutes on each side, or until lightly browned. Add the broth, the next 4 ingredients, the porcini mushrooms, and the mushroom roasting liquid in the roaster. Cut 1 onion into slices and bake in a roasting pan. Place a sheet of parchment paper directly on the brisket and cover the roaster with a tightly closed lid.Bake at 325 degrees for 4 hours or until the brisket is fork-soft. Let stand, covered with parchment and lid, in a roasting pan at room temperature for 30 minutes. Remove the parchment paper.Meanwhile, thinly slice the remaining 2 onions. Melt 2 tablespoons butter in a large skillet over medium-high heat. Add the onion slices and cook, stirring occasionally, for 15 minutes or until golden and soft. Remove from the pan.Increase the heat to medium-high; melt 2 tablespoons butter in a skillet. Add the cremini mushrooms and simmer for 8 minutes or until lightly browned and tender. Remove the cremini mushrooms from the pan; add to the onion mixture.Transfer the brisket to a cutting board and slice across the fibers. Transfer the slices to a serving platter and cover loosely to keep warm. Pour the cooking liquid through a fine wire mesh sieve into a medium-sized saucepan; remove any leftovers. Bring the liquid to a boil over medium-high heat. Add the onion and mushroom mixture and cook, stirring occasionally, for 5 minutes.Melt the remaining 2 tablespoons butter in a small saucepan over medium-high heat; add the cornstarch and beat until smooth. Cook the mixture, whisking constantly, for 30 seconds. Add the cornstarch mixture to the gravy; beat until smooth. Bring the gravy to a boil, whisking occasionally. Cook the gravy, whisking constantly, for 1 minute. Remove from heat; season to taste with kosher salt and freshly ground black pepper. Serve the gravy on top of the brisket.Note: We tested Swanson beef broth with 50% less sodium.