Season the beef cubes with salt and black pepper evenly.
Heat olive oil in a large skillet over medium-high heat.
Brown the beef cubes in batches, about 3-4 minutes per side, until all sides are seared. Transfer browned beef to the slow cooker.
In the same skillet, add sliced onion and cook for 3-4 minutes until softened.
Add minced garlic and cook for an additional 1 minute until fragrant.
Transfer the onion and garlic mixture to the slow cooker.
Pour beef broth, Worcestershire sauce, and tomato paste into the slow cooker. Stir to combine.
Add dried thyme and dried rosemary to the slow cooker and stir gently.
Cover and cook on low for 8 hours or until the beef is tender.
About 30 minutes before serving, whisk together the all-purpose flour and cold water in a small bowl until smooth.
Stir the flour mixture into the slow cooker to thicken the gravy.
Cover and cook on high for 30 minutes until the gravy has thickened.
Stir the gravy before serving and adjust seasoning with salt and pepper if needed.