Flat-cut brisket comes in a range of sizes. If you get a large, oblong piece, trim the thin end off, if necessary, so it will fit in a Dutch oven and cook evenly.
Stir together first 4 ingredients. Rub mixture over both sides of brisket. Place brisket in a parchment paper-lined jelly-roll pan; cover and chill 8 hours.
Let brisket stand at room temperature 30 minutes. Preheat oven to 325°. Soak dried porcini mushrooms in 2 cups boiling water 10 minutes.
Meanwhile, cook brisket in hot oil in a large Dutch oven 4 minutes on each side or until browned. Add broth, next 4 ingredients, porcini mushrooms, and mushroom soaking liquid to Dutch oven. Cut 1 onion into wedges, and add to Dutch oven. Place a piece of parchment paper directly on brisket, and cover Dutch oven with a tight-fitting lid.
Bake at 325° for 4 hours or until brisket is fork-tender. Let stand, covered with parchment and lid, in Dutch oven at room temperature for 30 minutes. Discard parchment paper.
Meanwhile, thinly slice remaining 2 onions. Melt 2 Tbsp. butter in a large skillet over medium heat. Add onion slices, and cook, stirring occasionally, 15 minutes or until golden and tender. Remove from skillet.
Increase heat to medium-high; melt 2 Tbsp. butter in skillet. Add cremini mushrooms, and sauté 8 minutes or until browned and tender. Remove cremini mushrooms from skillet; add to onion mixture.
Remove brisket to a cutting board, and slice across the grain. Transfer slices to a serving platter, and cover loosely to keep warm. Pour cooking liquid through a fine wire-mesh strainer into a medium saucepan; discard solids. Bring liquid to a boil over medium-high heat. Stir in onion-mushroom mixture, and cook, stirring occasionally, 5 minutes.
Melt remaining 2 Tbsp. butter in a small saucepan over medium-high heat; add cornstarch, and whisk until smooth. Cook mixture, whisking constantly, 30 seconds. Add cornstarch mixture to gravy; whisk to combine. Bring gravy to a boil, whisking occasionally. Cook gravy, whisking constantly, 1 minute. Remove from heat; season to taste with kosher salt and freshly ground black pepper. Serve gravy over brisket.
Note: We tested with Swanson 50% Less Sodium Beef Broth.