Combine broth, lemon juice, soy sauce, garlic, and liquid smoke in a roasting pan. Place brisket in the pan, fat side up, and drizzle some of the marinade over the top. Sprinkle liberally with Montreal steak seasoning, and place the onions along the sides of the brisket. Cover pan with foil, and marinate in the refrigerator for 24-48 hours. Preheat oven to 300 degrees, and place the brisket on the middle rack of the oven. Bake for about 40 minutes per pound of meat you are preparing. I baked my five pound brisket for 3 1/2 hours. The meat should be fork tender.

Transfer the brisket to a cutting board, and slice against the grain. Place the slices back into the marinade. This marinade is delicious drizzled on top of the meat and some mashed potatoes.