Coat the inside of a slow cooker with about 1 tablespoon olive oil.
Coat the inside of a skillet with about 1 tablespoon olive oil and heat over medium-high heat. Cook roast in the hot oil until browned, about 5 minutes per side. Cook and stir onion in the hot oil during the last 5 minutes of browning the roast. Transfer roast to the slow cooker, reserving onion in the skillet.
Pour water over onion; crumble bouillon cube into water. Add garlic and adobo seasoning to onion mixture; cook and stir until onion is tender, about 5 minutes. Pour mixture over roast.
Cook on High until meat is cooked through and tender, about 4 hours.
