Begin by preheating your oven to 325°F (163°C). If you're opting for a slow cooker, you can skip this step.

In a mixing bowl, combine the Lipton Onion Soup Mix, water, ketchup, garlic powder, black pepper, and brown sugar if you're using it. Stir until the mixture is well blended.

Place the brisket in your roasting pan or slow cooker. Pour the prepared sauce over the meat, ensuring it's thoroughly coated. Arrange the sliced onions on top of the brisket.

Oven Method: Cover the roasting pan loosely with aluminum foil and bake for approximately 3 hours, or until the brisket becomes tender. If you prefer a thicker gravy, you can remove the foil during the last 30 minutes of cooking.

Slow Cooker Method: Cover the slow cooker with its lid and cook on low heat for about 8 hours, or until the brisket reaches your desired tenderness.

Once cooked, transfer the brisket to a cutting board and let it rest for about 15 minutes. This resting period allows the juices to redistribute throughout the meat. After resting, slice the brisket against the grain to ensure maximum tenderness. Serve it with the flavorful onion sauce from the pan or slow cooker.