Finely dice the garlic. Chop the celery and carrots. Skin potatoes and chop into small bite-sized pieces or cubes.
Cut the beef into small cubes (3/4" to 1"), trimming visible fat. Season with salt and pepper
Coat skillet with non-stick cooking spray and sear the beef on med-high (about 2-3 minutes per side), just enough to sear and lightly brown the outside
Transfer beef to slow cooker and set to low
Add all vegetables to slow cooker
Add thyme, parsley, garlic, salt and pepper
Add beef stock, diced tomato, and Worcestershire sauce and stir
Cover and cook on low for 8 to 9 hours in slow cooker
Optional (for thickening sauce): About 30 minutes before serving, create slurry by stirring 1 - 2 Tbsp corn starch into 2 - 4 Tbsp water, then return to slow cooker, stir, cook on high final 30 minutes