Heat avocado oil in a large skillet over medium-high heat; cook beef roast in the hot oil until browned, about 4 minutes per side. Transfer roast to a slow cooker. Pour tomatoes and beef broth over roast; add onions and bell peppers.

Mix tomato paste, garlic, olive oil, vinegar, cumin, and salt together in a bowl; pour over onions and bell peppers.

Cook on High for 1 hour. Decrease heat to Low and cook until beef is tender enough to shred, about 8 hours.

Remove beef from slow cooker and shred on a work surface using 2 forks. Return shredded beef to slow cooker; add cilantro and stir.

Cook on Low for 1 hour more.