Spray 4-5 quart slow cooker with cooking spray. In a small bowl, mix salt, pepper, spices, and garlic. Rub seasoning mixture on beef pieces, set aside. Place onion in cooker, top with beef.
In medium bowl, mix vinegar, tomato paste and brown sugar. Pour mixture over beef. Cover, cook on low heat setting to 9-11 hours.
Remove beef from cooker. Transfer sauce to 2 quart saucepan, heat to boiling over medium-high heat, stirring occasionally until reduced to about half.
Cut beef against the grain into slices, place on serving platter. Serve with sauce.