Pat the sliced beef completely dry with paper towels. Place the beef and cornstarch in a large zip-top bag or bowl. Seal or cover and toss vigorously until every piece is lightly and evenly coated.

Heat the vegetable oil in a large skillet or wok over medium-high heat. Working in batches to avoid crowding, add the coated beef slices in a single layer. Sear for about 60-90 seconds per side until a golden-brown crust forms. Transfer seared beef to the bowl of your slow cooker.

In a medium bowl, whisk together all the sauce ingredients: soy sauce, water, brown sugar, hoisin, minced garlic, grated ginger, sesame oil, and red pepper flakes (if using). Pour the sauce over the seared beef in the slow cooker. Add the white parts of the scallions. Gently stir to combine.

Cover and cook on LOW for 4 hours. Do not stir during cooking. About 15 minutes before the end of cooking, stir in the green parts of the scallions.

If the sauce is thinner than you'd like, carefully ladle about 1/2 cup of the hot liquid from the slow cooker into a small bowl. Whisk in 1 tablespoon of cornstarch until smooth, then stir this slurry back into the slow cooker. Cook on HIGH, uncovered, for an additional 10-15 minutes until slightly thickened. Serve immediately over cooked rice in bowls, garnished with sesame seeds.