Preheat oven to 250 degrees.

Salt and pepper all sides of the London broil.

Combine red wine vinegar, ketchup, brown sugar, garlic, Worcestershire, and beef broth in a small bowl and set aside.

Heat a dutch oven over high heat and add butter and olive oil. Once the dutch oven is extremely hot, carefully add the London broil and sear on all sides (about 15 seconds on each side).

Remove dutch oven from heat. Add onions and pour sauce over the London broil. Cover and place in the oven for 2.5 hours.

Remove from oven and check on the meat, it should be fork-tender. If not, cover and cook for an additional 30 minutes or so.

When done, you can mix a tablespoon of cornstarch or flour with the juices from your pan to thicken the sauce and make a gravy. Serve with your favorite side dishes.

When you get to step 5: Transfer the London broil to a slow cooker instead of placing in the oven. Cook on low for 7-8 hours or on high for 3-4 hours.

After you sear the meat, transfer to your instant pot instead of the slow cooker. Cook on “high” or “manual” mode for 1 ½ hours. Let pressure release naturally when done (about 15 minutes). When done, shred beef with two forks.