Using a sharp knife, cut 12 deep slits all over the roast (both sides and ends). Stuff the garlic cloves into the slits. Sprinkle salt and pepper liberally all over the meat.
Spread half the sliced onions and oregano sprigs over the bottom of the slow cooker crock. Place the roast on top, and spread remaining onions and oregano over the meat.
Pour the tomato sauce over the roast, and then fill the empty sauce can with water and pour that around the meat. Cover and cook on low for 8-10 hours. Remove oregano stems before serving.