Season the flank steak with salt and black pepper on both sides.
In a bowl, whisk together the soy sauce, beef broth, tomato paste, honey, Worcestershire sauce, ground cumin, smoked paprika, and red pepper flakes.
Place the sliced onions and garlic cloves in the bottom of the slow cooker.
Lay the seasoned flank steak on top of the onions and garlic.
Pour the prepared sauce over the steak, ensuring that it is evenly coated.
Cover the slow cooker and cook on low for 7-8 hours, or on high for 3-4 hours, until the steak is tender.
Once cooked, carefully remove the steak from the slow cooker and transfer it to a cutting board. Let it rest for a few minutes.
Slice the steak across the grain into thin strips.
Serve the savory and spicy slow cooker flank steak over a bed of creamy mashed potatoes (if desired) spooning the flavorful sauce from the slow cooker over the steak.
Garnish with fresh parsley for a pop of color and added freshness. Enjoy!