1. Heat duck fat in a large soup pot over medium heat until melted. Add onions and cook, stirring occasionally, 10 minutes.
2. Sprinkle pork with salt and pepper. Add pork, garlic, and next 6 ingredients to pot; bring to a boil. Cover, reduce heat to low, and simmer 3 hours.
3. Remove pork from liquid with a slotted spoon, discarding bay leaves and celery and reserving liquid in pot, and transfer to a mixing bowl. Beat pork with an electric mixer at low speed until meat is shredded. Gradually stir in 3 to 3 1/2 cups reserved cooking liquid until mixture is somewhat smooth and blended. Season to taste with additional salt and pepper, if desired.
4. Divide mixture among canning jars. Skim any remaining fat from cooking liquid in pot; spoon fat over pork mixture. Cover and chill until firm.
5. Spread mustard on top halves of baguettes. Spread rillettes on bottom halves; top with pickle slices and baguette tops. Cut sandwiches in half, if desired. Store remaining rillettes in refrigerator up to 2 weeks.