In a skillet heat 1 tbsp of the oil over med-high heat. Add beef and cook until browned through. Remove with a slotted spoon and transfer to the slow cooker. Drain the liquid from the pan. Reduce the heat to med. Add the remaining oil to the pan. Add onions and celery, and cook until well softened. Add garlic, salt, thyme, and peppercorns stirring for 1 minute. Stir in tomato soup, stock, and Worcestershire sauce. Transfer to slow cooker, add rigatoni and stir well.
Cover and cook on low for 8 hours, or cook on high for 4 hours until very hot and bubbly. Stir in the cheese and cook on high for another 20 minutes, until the cheese is melted.