Step 1: Coat the beef - Add the thinly sliced flank steak to a large bowl and sprinkle the cornstarch over the top. Toss everything together until the beef is lightly coated. This step helps thicken the sauce later and gives the beef that glossy finish everyone loves.
Step 2: Sear the beef - Heat the oil in a skillet over medium high heat. Add the beef in batches and sear briefly on both sides. You are not cooking it through, just giving it a little color and flavor. This step is optional if you are short on time, but it adds a nice depth to the final dish.
Step 3: Add beef to the slow cooker - Transfer the seared beef into the slow cooker. Spread it out a bit so it cooks evenly.
Step 4: Mix the sauce - In a bowl, whisk together the soy sauce, brown sugar, beef broth, garlic, and ground ginger. Pour this mixture over the beef in the slow cooker. Sprinkle the shredded carrots on top.
Step 5: Cook low and slow - Cover and cook on low for 4 to 5 hours. The beef will become tender and the sauce will slowly thicken as it cooks.
Step 6: Garnish and serve - Once the beef is tender and the sauce is glossy, give everything a gentle stir. Garnish with green onions and scallions before serving. If you like sesame seeds, this is a good moment to sprinkle those on too.