Prepare the Beef: First, take your flank steak and slice it thinly against the grain. This helps make it extra tender. Then, put the sliced beef in a large zip-top bag or bowl and add the cornstarch. Toss it all together until the beef is evenly coated. This helps to thicken the sauce later.

Sear the Beef (Optional but Recommended): Heat the olive oil in a large skillet over medium-high heat. Add the beef and sear it for about 2-3 minutes per side, just until it's browned. You don't need to cook it all the way through. This step adds extra flavor, but you can skip it if you're short on time.

Combine Ingredients in the Slow Cooker: In the slow cooker, whisk together the soy sauce, water, brown sugar, minced garlic, ground ginger, and red pepper flakes (if using).

Add the Beef: Add the seared (or unseared) beef to the slow cooker, making sure it's submerged in the sauce.

Cook: Cover and cook on low for 4-6 hours, or on high for 2-3 hours. The beef is ready when it's very tender. The longer it cooks on low, the more tender it will become. So if you have the time, I always opt for the longer, slower cook. You can do a quick check by seeing if the beef is easily shredded with a fork.

Thicken the Sauce (Optional): If you want a thicker sauce, about 30 minutes before the end of the cooking time, take out about ½ cup of the sauce and whisk in 1 tablespoon of cornstarch until smooth. Pour this mixture back into the slow cooker and stir well.

Serve: Once the beef is cooked and the sauce is to your liking, it's time to serve! Garnish with sliced green onions and sesame seeds.