Heat vegetable oil in a skillet over medium-high heat. Add the sliced beef and sear the strips until browned on all sides, about 3-4 minutes. This step enhances the flavor but can be skipped if you prefer.
Transfer the browned beef to the crock pot.
Add the sliced onion, red bell pepper, green bell pepper, and minced garlic to the crock pot.
Pour in the diced tomatoes, soy sauce, Worcestershire sauce, ground black pepper, and salt. Stir everything together until well combined.
Cover the crock pot with a lid and set it on low heat for 6 hours, or until the beef is tender.
If you prefer a thicker sauce, mix the cornstarch with a little water to create a slurry. About 30 minutes before serving, add this mixture to the crock pot and stir until the sauce thickens.
Serve the pepper steak over cooked rice.