Prepare the Beef: Thinly slice the flank steak or sirloin against the grain into strips, about 1/4-inch thick. If you find it hard to slice the beef, freezing it for 20-30 minutes makes it easier.

Mix the Sauce: In a bowl, whisk together soy sauce, brown sugar, sesame oil, garlic, ginger, water, and rice vinegar (if using). If you want a thicker sauce, mix the cornstarch with 2 tablespoons of cold water to create a slurry, and add it to the sauce mixture.

Add Beef and Sauce to the Crockpot: Place the sliced beef in the bottom of the crockpot. Pour the sauce mixture over the beef and stir to coat evenly.

Cook the Beef: Cover the crockpot and cook on low for 4-5 hours, or on high for 2-3 hours, until the beef is tender.

Add the Broccoli: About 30 minutes before serving, add the broccoli florets to the crockpot, stirring to mix with the beef and sauce. If using frozen broccoli, add it at the same time as fresh.

Thicken the Sauce (Optional): If you prefer a thicker sauce, mix the cornstarch with cold water to form a slurry and stir it into the crockpot 10-15 minutes before serving. Allow the sauce to thicken.

Serve: Serve the Crockpot Beef and Broccoli over cooked rice. Garnish with sesame seeds if desired.