Add beef, mushrooms, onion, broth, teriyaki sauce and brown sugar in a slow cooker and mix well.
Heat on high for 3-4 hours or low for 6-8 hours.
In the last 30 minutes of cooking, dissolve corn starch in water and add to the slow cooker. Stir in broccoli.
Set slow cooker to high and heat for 30 minutes until broccoli is tender, stirring occasionally.