Slice onion in about 4 thick slices and arrange in a single layer on the bottom of the slow cooker.
Whisk together tomato paste, lime juice, adobo chilis and worcestershire sauce in a small bowl, set aside.
in another small dish, mix together the paprika, chili powder, oregano, cumin, salt and pepper.
Sprinkle the spice mixture all over the roast, pressing it into the meat so that it sticks.
Place seasoned roast on top of onions in slow cooker. Pour tomato paste mixture on top of roast, spreading to around to cover the top.
Cover and cook on low for about 7-8 hours.
Remove roast from slow cooker and use to forks to shred the beef. Return beef to slow cooker and toss in the liquid. Continue to cook on low for about 30 more minutes.