Pat the chuck roast pieces dry with paper towels. Season generously all over with salt and black pepper.

Heat olive oil in a large skillet over medium-high heat. Sear the beef pieces for 3-4 minutes per side until deeply browned. This adds significant flavor. Remove beef from skillet and set aside.

Add the chopped yellow onion to the same skillet (add a splash more oil if needed). Cook for 5-7 minutes until softened and translucent, scraping up any browned bits from the bottom of the pan. Add the minced garlic and cook for another 1 minute until fragrant.

Pour a splash (about 1/4 cup) of beef broth into the skillet, scraping the bottom to deglaze and incorporate all the flavorful browned bits. Transfer the sautéed onions and garlic to the slow cooker.

Place the seared beef into the slow cooker. In a medium bowl, whisk together the remaining beef broth, Worcestershire sauce, soy sauce, tomato paste, and dried thyme. Pour this mixture over the beef in the slow cooker. Add the bay leaves.

Cover the slow cooker and cook on LOW for 7-8 hours, or on HIGH for 3-4 hours, until the beef is fork-tender and shreds easily.

Carefully remove the beef from the slow cooker and place it on a cutting board or large platter. Remove and discard the bay leaves. Skim off any excess fat from the liquid in the slow cooker, if desired.

In a small bowl, whisk together the cornstarch and cold water to create a slurry. Turn the slow cooker to HIGH (if not already there). Whisk the cornstarch slurry into the cooking liquid in the slow cooker. Cook for another 15-30 minutes, stirring occasionally, until the gravy has thickened to your desired consistency.

While the gravy thickens, use two forks to shred the cooked beef into bite-sized pieces. Return the shredded beef to the thickened gravy in the slow cooker and stir to combine. Serve the Slow Cooker Beef Manhattan hot, traditionally piled high on slices of white bread or dinner rolls, and often accompanied by mashed potatoes.