Trim chuck roast for any excessive fat, rinse and pat dry with paper towels. Then season salt and pepper the chuck roast.
Generously rub the roast with half of the onion and garlic powder mixture. You may refrigerate until ready to use. This part can be done ahead and place in the fridge for up to 24 hours before proceeding with the next steps.
Heat about 1 tablespoon oil in skillet on medium high heat. Next, place chuck roast onto skillet and sear meat for about 3-4 minutes or until brown. Flip roast on the other side, add smashed garlic and butter, and continue cooking on medium for about 2-3 more minutes.
Transfer the roast to the insert of a slow cooker together with the garlic (I used a 6-quart slow cooker ).
Meanwhile, pour in beef broth or water, Worcestershire sauce and Pepperoncinis sauce to the same cast iron or skillet, scrape off any bits from the skillet. Add the beef bouillon, remaining garlic and onion powder and cornstarch; give it a good stir. Pour to the slow cooker insert.
Add pepperoncinis, green onions, bay leaf and parsley, together with carrots and potatoes, if using any.
Cover and slow cook on high for about 4 – 5 hours or low for 8- 9 hours. Start checking after 3 1/2 hours, until the roast is tender.