Recipe: Spicey Beef Brisket (CrockPot/SlowCooker)

Gail, here is a recipe recently posted in TKL chat. It sounds like what you are wanting.
It does not specifically call for onions, but I sure would throw them in!!! Hope this helps.

Spicy Beef Roast
Source: Rival Crockpot Cookbook

3 pounds round tip roast, or brisket trimmed of fat
2 tablespoons cracked black peppercorns
2 cloves garlic, minced
3 tablespoons balsamic vinegar
1/4 cup reduced-sodium soy sauce
2 tablespoons Worcestershire sauce
2 teaspoons dry mustard

Rub cracked pepper and garlic onto roast. Put roast in crockpot. Make several shallow slits in top of roast. In a small bowl, combine remaining ingredients and pour over meat. Cover; cook on low 8-10 hours (high 4-5 hours).