Season pork with salt and pepper. In a Dutch oven, heat oil over medium-high heat; add pork and cook until brown all over. Transfer to slow cooker.
In the Dutch oven, add onions, garlic, chili powder, coriander and bay leaves; cook, stirring often, until onions are softened, about 5 minutes.
Add tomato paste and cook, stirring for 2 minutes. Add tomato sauce, sugar, vinegar and Worcestershire sauce, scraping up any browned bits. Pour mixture all over the pork in slow cooker. Cover and cook on low until pork is tender, 8 to 10 hours.
Transfer pork to cutting board and let stand for 10 minutes. Using 2 forks, shred or “pull” pork. Set aside.
Meanwhile, pour liquid from slow cooker into large saucepan; skim off fat. Bring to a boil over high heat; boil until mixture has been reduced to 3 cups, about 10-12 minutes. Discard bay leaves.
Add pulled pork into the sauce and stir to coat. Sprinkle with green onions.