Lightly spray a 6-8 quart crock pot with non-stick cooking spray.
Add meat to a bowl or a large sealable gallon baggie.
Sprinkle with cornstarch and stir or shake to coat.
Place in the crock pot, and add diced onions.
In a medium bowl, add soy sauce, water, brown sugar, sesame oil, garlic, ginger and red pepper flakes.Stir to combine and pour over the beef and onions. Stir.
Cover and cook on high for 3-4 hours or on low for 5-6 hours, or until the meat is tender.
In the last hour of cooking, add broccoli, stir to combine, cover and continue to cook until done.
Top with green onions and sesame seeds, if desired.
Serve over rice, mashed potatoes, or noodles.