Place the beef tips, frozen pearl onions, tomato paste, garlic powder, onion powder, dried thyme, salt, and pepper in a 6- to 7-quart slow cooker. Stir to coat.
Add the sliced mushrooms and pour the beef broth over everything.
Cover the slow cooker and cook on high for 4 hours or low for 6-8 hours, until the beef is tender.
To thicken the gravy, create a cornstarch slurry: Mix 2 tablespoons of cornstarch with ¼ cup of the hot liquid from the slow cooker in a small bowl, whisking until smooth.
Pour the slurry back into the slow cooker and stir until incorporated. Cover and cook on high for 30 minutes or until the gravy thickens.
If you'd like thicker gravy, repeat the slurry step with an additional 2 tablespoons of cornstarch.
Serve over mashed potatoes, parsnips, or cauliflower, and garnish with parsley if desired.