Pat roast dry with paper towels.
Make 12 deep incisions in roast and stuff with garlic cloves.
Drizzle roast with olive oil and season with 2 tsp salt, 1 1/2 tsp pepper, 1 Tbsp garlic powder, and minced herbs.
Heat olive oil in cast iron skillet over medium-high heat and sear roast 5-6 minutes per side until golden brown.
Layer half the onions, carrots, celery, smashed garlic, and potatoes in slow cooker.
Place seared roast on top and arrange remaining vegetables and potatoes around it.
Tuck herb sprigs around roast.
Whisk together beef broth, Worcestershire sauce, balsamic vinegar, tomato paste, 1 1/2 tsp salt, and 3/4 tsp pepper.
Pour liquid mixture into slow cooker until it reaches halfway up the roast.
Cover and cook on LOW for 9 hours until meat shreds easily with a fork.
Transfer roast to cutting board and shred with two forks.
Arrange shredded meat on platter with vegetables.
Strain cooking liquid into saucepan and skim off excess fat.
Bring liquid to low simmer.
Whisk cornstarch and water together to make slurry.
Slowly whisk slurry into simmering liquid and cook 2-3 minutes until thickened.
Garnish with chopped parsley and serve.