Heat a skillet over medium-high heat and brown the sausages for about 2 minutes per side. Remove and set aside.
In the same skillet, melt butter and cook the sliced onions for about 5 minutes until softened.
Add the onions to the slow cooker and place the browned sausages on top.
Pour in the beef broth and Worcestershire sauce. Add garlic powder, onion powder, salt, and black pepper.
Cover and cook on LOW for 6 hours or HIGH for 3 to 4 hours.
In a small bowl, mix cornstarch and water until smooth.
Stir the cornstarch mixture into the slow cooker during the last 20 minutes of cooking to thicken the gravy.
Garnish with fresh parsley and serve hot over mashed potatoes, noodles, or rice.