In a large ziplock bag, mix the cornstarch adobo seasoning and a pinch of salt and black pepper. Add the stew meat and shake and massage the bag until all of the meat is evenly coated.
Heat a large skillet over medium high heat, or dutch oven. Add the oil to the skillet. In batches, brown the meat on all sides. Place the meat into the slow cooker. Add the sliced onion and cook in the pan to soften, scraping the bottom to help deglaze the skillet.
Add the garlic to the skillet and heat through. Then, add a bit of the beef broth, scraping the bottom to remove any remaining pieces. Pour the contents of this skillet into your slow cooker.
To the slow cooker, also add the tomato sauce, potatoes, carrots, and all remaining seasonings. Stir to combine. Cover the slow cooker and cook on low for 6 to 8 hours or until the beef is tender.