Heat olive oil in a large skillet over medium-high heat.
Add beef cubes in batches and sear until browned on all sides, about 3-4 minutes per batch. Transfer browned beef to the slow cooker.
In the same skillet, add diced onion and cook for 2-3 minutes until softened.
Add minced garlic and cook for 30 seconds until fragrant.
Pour in beef broth, soy sauce, Worcestershire sauce, and brown sugar. Stir to combine and bring to a simmer for 2 minutes.
Add smoked paprika, black pepper, salt, and crushed red pepper flakes to the skillet. Stir well.
Pour the sauce mixture over the beef in the slow cooker.
Cover and cook on low for 6 hours or until beef is tender.
About 15 minutes before serving, mix cornstarch and cold water in a small bowl until smooth.
Stir the cornstarch slurry into the slow cooker to thicken the sauce.
Cover and cook on high for 15 minutes until sauce has thickened.
Serve the beef bites hot with your choice of side dishes.