Pat the roast dry, then season all over with salt and pepper.
Heat oil in a large skillet over medium-high. Sear beef on all sides until deeply browned, 2–3 minutes per side.
Whisk broth, tomato paste, vinegar, brown sugar, chili powder, cumin, and oregano in the slow cooker.
Stir in the chopped onion and minced garlic. Nestle the seared roast into the sauce and turn to coat.
Cover and cook on LOW for about 8 hours (or HIGH for 4–5) until the beef is fork-tender.
Transfer beef to a board. Shred with two forks, discarding large fatty pieces.
Skim fat from cooking liquid. For thicker sauce, stir in cornstarch slurry and simmer in the cooker until glossy.
Return shredded beef to the sauce and toss. Taste and adjust salt, pepper, or a splash more vinegar.