Heat the oil in a large stock pot over medium heat. Add the stew meat and cook until browned, about 5-7 minutes. Remove meat from pan and set aside. Do not pour out the drippings.
Add the onion, bell peppers and mushrooms to the pot and sauté until tender, 3-5 minutes.
Add the garlic and sauté for 30 seconds.
Add the broth the pot and scrape up any browned bits from the bottom of the pan.
Return the meat to the pot.
Add the peppers, butter, and brown gravy mix. Stir to combine.
Cover and reduce the heat to low. Simmer for 1 ½ - 2 hours.
Remove the pepperoncini peppers before serving and discard.
Season with salt and pepper to taste. Serve over hot buttered egg noodles, rice or mashed potatoes.