Pat the brisket dry with paper towels to help the seasonings adhere better. Rub the garlic salt and meat tenderizer evenly over all sides of the brisket, then finish with the freshly cracked black pepper. This creates a flavorful crust that will develop during cooking. Place the seasoned brisket in a 9x13 inch baking dish or similar roasting vessel with the fat cap facing up.

In a medium bowl, whisk together the beef gravy, Italian dressing, and Worcestershire sauce until smooth and well combined. Add the onion soup mix and stir thoroughly to distribute it evenly throughout the liquid—this ensures the salt and flavors are balanced. Gently fold in the thinly sliced mushrooms, being careful not to break them apart. I find that slicing the mushrooms uniformly helps them cook evenly and distribute their umami flavor throughout the braising liquid.

Preheat your oven to 275°F. Pour the braising liquid from Step 2 evenly over the seasoned brisket, making sure the mushrooms are distributed throughout the dish. Cover the baking dish tightly with aluminum foil to trap steam and create a moist braising environment. Bake for 4 to 5 hours at 275°F until the brisket is fork-tender and easily shreds when tested. I like to check the brisket around the 3.5-hour mark—if it's already tender, you can pull it out a bit early rather than overcooking it, which can dry out the meat.

Remove the brisket from the oven and let it rest for 10 minutes while still covered with foil. This allows the juices to redistribute throughout the meat, ensuring it stays moist and tender when sliced. Carefully slice or shred the brisket against the grain, and serve it with the rich mushroom gravy spooned over the top.