Layer half of the onions and peppers in the bottom of your slow cooker. Lay flank steak on top and season with salt and pepper.
Top with the remaining onions, peppers, garlic, tomatoes, and seasoning.
Cook on low for 7 to 8 hours (or on high for 3 to 4 hours) or until the flank steak can be easily shredded.
Remove steak from the crockpot and shred using 2 forks .
Return meat back to the slow cooker for another 15 minutes to allow the meat to absorb the juices. Serve meat over cauliflower rice, roasted vegetables, or lettuce cups.