Spray 3 1/2- to 4-quart slow cooker with cooking spray. In cooker, place onion; top with pork and remaining ingredients except tomatoes and buns.
Cover; cook Low heat setting 7 to 9 hours.
Remove pork from cooker to cutting board with slotted spoon. Shred pork with 2 forks; return to cooker and mix well. Place tomatoes on bottom s of buns. Using slotted spoon, add 1/3 cup pork over tomatoes; top with tops of buns.