Prepare the Beef: Season the flank steak generously with salt and pepper. Heat a large skillet over medium-high heat and add a drizzle of oil. Sear the steak for about 3-4 minutes on each side until nicely browned. Transfer to a plate and set aside.

Build the Base: In the same skillet, lower the heat to medium and add chopped onions and sliced bell peppers. Sauté for about 5-7 minutes until softened. Add minced garlic and cook for an additional minute.

Combine Ingredients: In a slow cooker or large pot, add the seared flank steak and sautéed vegetables. Pour in the tomato sauce, cumin, vinegar, and any optional ingredients. Stir gently to combine thoroughly.

Cook: Cover with a lid and set on low heat. If using a slow cooker, cook for about 6-8 hours, or simmer on low for 4-5 hours on the stovetop.

Finish: Once the beef is tender, shred it into bite-sized pieces with two forks. Return to the pot and mix with the sauce. Let it rest for a few minutes before serving.