Instructions:

1. In a large bowl, coat the thinly sliced beef with cornstarch, making sure each piece is well coated. Set aside.

2. Heat the vegetable oil in a skillet or frying pan over medium-high heat. Add the beef slices in batches and cook until browned on both sides. Transfer the browned beef to the slow cooker.

3. In the same skillet, add the minced garlic and grated ginger. Sauté for about 1-2 minutes until fragrant.

4. In a separate bowl, whisk together the soy sauce, water, brown sugar, and red pepper flakes. Pour the sauce mixture into the skillet with the garlic and ginger. Bring it to a simmer and cook for 2-3 minutes until the sauce thickens slightly.

5. Pour the sauce over the beef slices in the slow cooker. Stir to ensure all the beef is coated with the sauce.

6. Cover the slow cooker and cook on low heat for 4-6 hours or on high heat for 2-3 hours. The beef should become tender and flavorful.