Turn slow cooker on to low heat.
Add sliced beef and onions.
In a medium sized bowl whisk together beef broth, soy sauce, dry sherry or oyster sauce, sesame oil, brown sugar, ginger, garlic and crushed red pepper flakes. Taste mixture and add salt and pepper accordingly.
Pour mixture over sliced beef and onions in your slow cooker. Gently toss to combine.
Cover and cook on low for 90 minutes or until meat is almost cooked.
Remove about 1/2 cup of liquid from your slow cooker and pour into a bowl.
Add cornstarch and stir until well blended.
Then return mixture to slow cooker and gently stir. Taste the mixture and decide if you think it needs more salt, pepper, garlic, ginger, sugar or honey. Once you are satisfied with the taste move on to the next step.
Add remaining vegetables, between 4 and 6 cups worth, and turn slow cooker up to high for between 20 to 30 minutes.
In the meantime, prepare rice or noodle of your choice, according to package directions.
Check to make sure vegetables are tender enough and sauce has thickened. Serve beef and vegetable mixture on top. Add toasted sesame seeds and green onions if desired. Serve immediately. Enjoy!