Place trimmed brisket into a large baking pan. (I use a Dutch oven)
Mix ketchup or chili sauce with water and onion soup mix. Pour sauce over the meat, top with sliced onions, if using, and season with salt and pepper. Cover pan with foil or lid, and marinate overnight in the fridge.
Preheat oven to 275 degrees. Bake brisket covered for 6 to 8 hours, depending on how large the brisket is. Check for tenderness after brisket has cooked for about 5-6 hours. Meat should be fork tender and come apart easily, if not, cook for 1-2 hours more.
When meat is done, remove from the oven, let rest for 15-20 minutes. On a cutting board, slice the brisket against the grain. Return the sliced meat to the sauce until ready to serve, or place meat on a platter and spoon sauce over the top.
Serve with buttered mashed potatoes and onions, if using.