Heat the oil in a large stock pot over medium high heat.
Season the meat generously with salt and pepper. Add the meat to the oil and cook until browned, about 5-7 minutes. You may need to brown the beef in batches to get a good sear. If so, drain any accumulated grease between batches.
Add the seared meat to the slow cooker.
Add the onion, bell pepper, mushrooms and garlic.
Whisk together the the broth and brown gravy mix. Pour the broth over the top.
Add the pepperoncini peppers and butter to the top.
Cook on low for five hours or on high for three hours.
Remove the lid and spoon out a few tablespoons of the cooking liquid into a small mixing bowl. Whisk in the cornstarch until smooth. Stir the cornstarch slurry into the beef tips.
Replace the lid and continue to cook for one more hour.
Remove the pepperoncini peppers before serving. Discard the peppers or remove the stems, chop them up and add them back into the pot.
Season the beef tips with salt and pepper to taste. Serve over hot egg noodles, rice or mashed potatoes.
Notes
Storage. Slow cooker beef tips with gravy should be stored in an airtight container in the refrigerator within two hours. Consume within 3-5 days.Freezing. Transfer the beef tips to an airtight freezer container or freezer bag. Be sure to leave some room for expansion. If using a freezer bag, fold the top of the bag over to remove excess air. Consume within three months.Reheating. If frozen, allow the beef tips to thaw in the refrigerator overnight. Transfer to a medium saucepan, cover and simmer over medium low heat until the gravy is bubbling and the beef tips are heated through. Cooking over low heat rather than high heat prevents the beef tips from becoming overcooked and mushy.