Coat the inside of the slow cooker lightly with cooking spray. Set aside.
Heat the oil in a large skillet over medium-high heat. Season the flank steak with salt and pepper, then sear lightly on both sides. Transfer the steak to the prepared slow cooker.
In the same skillet, cook the red and green bell peppers and onion for 2-3 minutes, or until tender. Add the garlic and cook for 1 additional minute, or until fragrant. Transfer the vegetables to the slow cooker, surrounding the steak.
In a small bowl, whisk together the tomato paste, beef broth, lime juice, cumin, oregano, and smoked paprika. Pour the mixture over the steak in the slow cooker.
Pour the diced tomatoes (with their juice) over the steak and sauce.
Cover and cook on low for 6-8 hours or high for 3-4 hours, or until the meat is tender and can be easily shredded with a fork.
Remove the steak from the slow cooker, shred it with two forks, and return it to the slow cooker. Add the kalamata olives, if using, and stir to combine.
Transfer the shredded steak to a serving platter and garnish with chopped cilantro, if desired. Serve warm.