Cut the flank steak into 2 or 3 large pieces that will fit comfortably in your slow cooker. Season the meat with salt and ground black pepper.
Heat about 2 tablespoon of oil in a skillet over medium high heat. Working in batches if needed, add the steak and sear the meat on both sides for about 5 -6 minutes or until browned. Transfer the meat to a plate.
Place the peppers and onions on the bottom of the slow cooker. Place the seared flank steak on top of the vegetables.
Stir in the garlic, tomato sauce, tomatoes, jalapeno, oregano, cumin, sugar, paprika, and salt. Gently mix to combine.
Cook on low for 8 hours hours or on high for 4.5 to 5 hours until the beef is tender and shreds easily with 2 forks.
Remove the beef from the slow cooker and using 2 forks, pull apart into thin, long shreds.
Return the shredded beef into the pot and mix to combine. Add the chopped cilantro or parsley and mix well. Season with salt and ground black pepper to taste and cook on low for an additional 20 minutes. Serve garnished with extra cilantro or parsley if desired.