Mix the onions, garlic, spices, and oil into a microwave-safe bowl. Place the bowl in the microwave and heat on high for 5-6 minutes. The onions and garlic should be softened and the ingredients should be aromatic.
While the aromatics are cooking place the chuck roast into the insert of the slow cooker.
Stir the tomato sauce and sugar into the bowl after the garlic and onions finish cooking. Pour the sauce over the chuck roast.
Tightly place the cover on the slow cooker and set the cooking time to 8-10 hours on low or 6-7 hours on high.
Place the beef in a bowl and shred. Toss with 1/2 - 1 cup or more of the braising liquid left over in the slow cooker. This adds flavor and will keep the meat moist. Stir in the lime juice.
Add salt and pepper to taste. Start with 1 teaspoon of both salt and pepper. Then add more, if needed.