In a large skillet or Dutch oven, melt 2 tablespoons of the butter over medium high heat. When it is shimmering and frothy, add the beef cubes in a single layer (work in batches if needed, do not crowd the pan). Let them cook undisturbed for 2 to 3 minutes per side, until they are deeply browned. Transfer the browned beef to your slow cooker.

Reduce the heat to medium. Add the remaining 2 tablespoons of butter to the same skillet. Add the sliced onion and cook for 4 to 5 minutes, until it is soft and translucent. Add the minced garlic and cook for just 30 seconds more, until fragrant.

Pour the beef broth into the skillet and use a wooden spatula to scrape up all those lovely brown bits stuck to the bottom. Stir in the Worcestershire sauce, soy sauce, smoked paprika, and dried thyme. Let it simmer for one minute.

Pour the onion and broth mixture over the beef in the slow cooker. Stir gently to coat all the beef bites.

Cover the slow cooker and cook on LOW for 6 to 7 hours or on HIGH for 3 to 4 hours. The beef is ready when it is fork tender and shreds easily.

In the last 30 minutes of cooking, if you want a thicker, more luscious sauce, whisk the cornstarch and cold water together in a small bowl to form a slurry. Gently stir it into the slow cooker. Replace the lid and cook for the remaining 30 minutes. The sauce will become glossy and rich.

Taste the sauce and adjust salt and pepper if needed. Garnish with fresh parsley. Serve the high protein slow cooker garlic butter beef bites hot, with your choice of sides. The meat should be so tender it almost melts.