Heat 2 tablespoons oil in large skillet. Add meat, working in batches if necessary and brown. Transfer to slow cooker.
Add additional oil and cook onions, peppers, and garlic for several minutes until slightly softened, stirring now and then. Add to the slow cooker.
Add tomato paste to the pan and cook for a minute or so, stirring. Add the stock to the pan, then stir in salt, oregano, cumin & pepper. Add the mixture to slow cooker along with vinegar and bay leaves.
Cover and cook on low for 8 hours or so, until very tender and shreddable. Using two forks, shred meat into the sauce.
Taste and adjust for salt, pepper, and vinegar to your liking.
Serve over rice.
Keywords: Bargain Meal of the Week, Beef, Bell Peppers, Chuck Roast, Cuban, Freezes Well, Slow Cooker, Vinegar