Heat the vegetable oil in a large skillet over medium-high heat. Sear the sliced flank steak in batches until browned on both sides, about 2-3 minutes per side.

Transfer the seared steak to the crock pot. Add the sliced onion, bell peppers, and minced garlic on top of the beef.

In a small bowl, mix together the soy sauce and Worcestershire sauce. Pour this mixture over the contents in the crock pot.

Cover and cook on low for 7-8 hours or on high for 4-5 hours, until the steak is tender and cooked through.

In the last 30 minutes of cooking, mix the cornstarch with water and stir it into the crock pot to thicken the sauce.

Once finished, taste and adjust with salt and pepper as necessary before serving with rice or noodles.