Pat dry the chuck roast and season liberally on all sides with salt and pepper.
Heat oil in a skillet over medium heat and brown the chuck roast on all sides until a golden brown crust has formed. (You can skip this step if in a hurry but there's a ton of flavor being developed if you have the time to brown your beef.)
Transfer the roast to the slow cooker. Add the onions and carrots to the pan.
Cook the onions and carrots for 3-4 minutes. Stir in the garlic and cook for 1 minute. Transfer to the slow cooker.
In a small bowl, whisk together the onion powder, garlic powder, pepper, salt, pepper, buttermilk, dill, chives and italian seasoning.
Sprinkle the spice mixture over the beef evenly.
Add the beef stock, peppers, bay leaf, worcestershire sauce and 1/4 cup of the pepperoncini juice to the slow cooker and stir to combine.
Cover and cook on high for 4-5 hours or low for 6-7 hours.
Mix the cornstarch into the room temperature butter to form a paste.
Remove the pot roast from the pot and cover to keep warm. Using a ladle, skim the fat from the broth or transfer the broth to a fat separator with a spout and remove the fat that way.
Stir the cornstarch mixture into the broth and cook for about 10 minutes or until slightly thickened.
Return the pot roast to the slow cooker with the gravy and shred with a fork, if desired. (We like to break off big hunks of meat and serve it that way.)
Serve immediately with the gravy over noodles, mashed potatoes or rice.
Video
Notes
Mississippi Pot Roast can be made up to 24 hours in advance. Store in the refrigerator and reheat either in the oven or slow cooker until hot.